pour faire fonctionner une ancienne version de R sur ma machine

Si vous pouviez m'aider à trouver une solution à une question simple :

Je crois que ma machine ubuntu a installé automatiquement R 3.6.0 : lorsque je tape : > R. il est écrit :

R version 3.6.0 (2019-04-26) -- "Planting of a Tree" Copyright (C) 2019 The R Foundation for Statistical Computing Platform: x86_64-pc-linux-gnu (64-bit)

Cependant, j'ai besoin d'utiliser une version antérieure de R, à savoir R 3.5, qui a été installée et a fonctionné sur ma machine Ubuntu, et je peux voir beaucoup de dossiers dans le répertoire (une longue liste suit ci-dessous) :

/home/bogdan/R/x86_64-pc-linux-gnu-library/3.5

Pourriez-vous m'indiquer comment revenir à R 3.5 au lieu d'utiliser R 3.6 ? Merci beaucoup,

bogdan

ps : la liste des dossiers dans ~/R/x86_64-pc-linux-gnu-library/3.5 est composée de 453 éléments.

abind/
acepack/
ALL/
alphahull/
amap/
annotate/
AnnotationDbi/
AnnotationFilter/
AnnotationForge/
apcluster/
ape/
aroma.light/
askpass/
assertthat/
backports/
base64enc/
bbmle/
beachmat/
beeswarm/
BH/
bibtex/
bindr/
bindrcpp/
Biobase/
BiocFileCache/
BiocGenerics/
biocGraph/
BiocInstaller/
BiocManager/
BiocNeighbors/
BiocParallel/
BiocStyle/
BiocVersion/
biocViews/
biomaRt/
Biostrings/
biovizBase/
bit/
bit64/
bitops/
bladderbatch/
blob/
bookdown/
brew/
broom/
BSgenome/
Cairo/
callr/
car/
carData/
Category/
caTools/
CCA/
CCP/
cellranger/
cellrangerRkit/
checkmate/
circlize/
cli/
clipr/
clisymbols/
coda/
colorspace/
combinat/
ComplexHeatmap/
contfrac/
corpcor/
corrplot/
cowplot/
crayon/
crosstalk/
cubature/
curl/
cvTools/
data.table/
DBI/
dbplyr/
DDRTree/
DelayedArray/
DelayedMatrixStats/
deldir/
densityClust/
DEoptimR/
desc/
DESeq/
DESeq2/
deSolve/
destiny/
devtools/
dichromat/
digest/
diptest/
distillery/
doBy/
docopt/
doParallel/
doRNG/
doSNOW/
dotCall64/
dplyr/
DropletUtils/
dtw/
dynamicTreeCut/
e1071/
EDASeq/
edgeR/
ellipse/
ellipsis/
elliptic/
EnrichmentBrowser/
enrichR/
EnsDb.Hsapiens.v86/
EnsDb.Mmusculus.v79/
ensembldb/
evaluate/
extRemes/
fansi/
fastcluster/
fastICA/
fda/
fields/
fitdistrplus/
fit.models/
flexmix/
FNN/
forcats/
foreach/
formatR/
Formula/
fpc/
fs/
futile.logger/
futile.options/
gage/
gbRd/
gdata/
genefilter/
geneplotter/
generics/
GenomeInfoDb/
GenomeInfoDbData/
GenomicAlignments/
GenomicFeatures/
GenomicRanges/
GEOquery/
GetoptLong/
GGally/
ggbeeswarm/
ggbio/
ggdendro/
ggfortify/
ggplot2/
ggrepel/
ggridges/
ggthemes/
gh/
git2r/
githubinstall/
Glimma/
GlobalOptions/
glue/
gmodels/
GO.db/
goftest/
googleVis/
GOplot/
GOstats/
gplots/
graph/
graphite/
gridExtra/
GSA/
GSEABase/
gtable/
gtools/
hash/
haven/
HDF5Array/
hdf5r/
hexbin/
highr/
Hmisc/
hms/
HSMMSingleCell/
htmlTable/
htmltools/
htmlwidgets/
httpuv/
httr/
hwriter/
hypergeo/
ica/
igraph/
impute/
ini/
inline/
IRanges/
irlba/
iterators/
jsonlite/
kBET/
KEGGgraph/
KEGGREST/
kernlab/
knitr/
labeling/
laeken/
lambda.r/
lars/
later/
latticeExtra/
lazyeval/
limma/
Linnorm/
lle/
lme4/
Lmoments/
lmtest/
locfit/
loo/
lsei/
lubridate/
M3Drop/
magrittr/
maps/
maptools/
markdown/
MAST/
Matrix/
MatrixModels/
matrixStats/
mclust/
MCMCglmm/
memoise/
metap/
mime/
minqa/
mixtools/
mnormt/
mockery/
modelr/
modeltools/
moments/
monocle/
munsell/
Mus.musculus/
mvoutlier/
mvtnorm/
NADA/
nloptr/
npsurv/
numDeriv/
openssl/
openxlsx/
OrganismDbi/
org.Hs.eg.db/
org.Mm.eg.db/
orthopolynom/
ouija/
packrat/
pathview/
pbapply/
pbkrtest/
pcaMethods/
pcaPP/
pcaReduce/
penalized/
permute/
PFAM.db/
pheatmap/
pillar/
pkgbuild/
pkgconfig/
pkgload/
pkgmaker/
PKI/
plogr/
plotly/
pls/
plyr/
plyranges/
png/
polyclip/
polynom/
prabclus/
praise/
preprocessCore/
prettyunits/
processx/
progress/
promises/
ProtGenerics/
proxy/
pryr/
ps/
purrr/
qlcMatrix/
quantreg/
R6/
randomForest/
ranger/
RANN/
rappdirs/
RBGL/
rcmdcheck/
RColorBrewer/
Rcpp/
RcppAnnoy/
RcppArmadillo/
RcppEigen/
RcppProgress/
RCurl/
Rdpack/
readr/
readxl/
refGenome/
registry/
reldist/
rematch/
remotes/
ReportingTools/
reprex/
reshape/
reshape2/
reticulate/
Rgraphviz/
rhdf5/
Rhdf5lib/
rio/
rJava/
rjson/
RJSONIO/
rlang/
RMariaDB/
rmarkdown/
R.methodsS3/
Rmisc/
RMTstat/
rngtools/
robCompositions/
robust/
robustbase/
ROCR/
R.oo/
Rook/
rprojroot/
rrcov/
Rsamtools/
rsconnect/
RSQLite/
rstan/
rstudioapi/
rsvd/
rtracklayer/
Rtsne/
RUnit/
R.utils/
RUVSeq/
rvest/
S4Vectors/
safe/
SC3/
scales/
scater/
scatterplot3d/
scde/
scfind/
scImpute/
scmap/
SCnorm/
scran/
scRNAseq/
scRNA.seq.funcs/
SDMTools/
segmented/
selectr/
sessioninfo/
Seurat/
sgeostat/
shape/
shiny/
ShortRead/
SingleCellExperiment/
slam/
SLICER/
smoother/
snow/
snowfall/
sourcetools/
sp/
spam/
SparseM/
sparsesvd/
spatstat/
spatstat.data/
spatstat.utils/
SPIA/
splancs/
sROC/
StanHeaders/
statmod/
stringi/
stringr/
SummarizedExperiment/
sva/
sys/
tensor/
tensorA/
testthat/
tibble/
tidyr/
tidyselect/
tidyverse/
tinytex/
topGO/
trimcluster/
tripack/
truncnorm/
TSCAN/
tsne/
TTR/
TxDb.Mmusculus.UCSC.mm10.ensGene/
TxDb.Mmusculus.UCSC.mm10.knownGene/
UpSetR/
usethis/
utf8/
VariantAnnotation/
vcd/
vegan/
VennDiagram/
Vennerable/
venneuler/
VGAM/
VIM/
vipor/
viridis/
viridisLite/
WGCNA/
whisker/
withr/
WriteXLS/
xfun/
XML/
xml2/
xopen/
xtable/
xts/
XVector/
yaml/
zCompositions/
zip/
zlibbioc/
zoo

ps2 : dois-je faire :

$apt-cache policy r-base

r-base:
  Installed: 3.6.0-1xenial
  Candidate: 3.6.0-2xenial
  Version table:
     3.6.0-2xenial 500
        500 https://cloud.r-project.org/bin/linux/ubuntu xenial-cran35/ Packages
 *** 3.6.0-1xenial 500
        500 https://cloud.r-project.org/bin/linux/ubuntu xenial-cran35/ Packages
        100 /var/lib/dpkg/status
     3.5.3-1xenial 500
        500 https://cloud.r-project.org/bin/linux/ubuntu xenial-cran35/ Packages
     3.5.2-1xenial 500
        500 https://cloud.r-project.org/bin/linux/ubuntu xenial-cran35/ Packages
     3.5.1-2xenial 500
        500 https://cloud.r-project.org/bin/linux/ubuntu xenial-cran35/ Packages
     3.5.1-1xenial 500
        500 https://cloud.r-project.org/bin/linux/ubuntu xenial-cran35/ Packages
     3.5.0-1xenial 500
        500 https://cloud.r-project.org/bin/linux/ubuntu xenial-cran35/ Packages
     3.2.3-4 500
        500 http://archive.ubuntu.com/ubuntu xenial/universe amd64 Packages
        500 http://archive.ubuntu.com/ubuntu xenial/universe i386 Packages