I can\'t access many Bioconductor packages in R 3.1.1 and I am quite disappointed with that. How can I downgrade from R 3.1.1 to R 3.0.2 or to some other version?
No
As point by @Deleet, this is FOR WINDOWS ONLY.
For the rest of the platforms, see: https://support.rstudio.com/hc/en-us/articles/200486138-Changing-R-versions-for-RStudio-desktop
Go to: Tools > Global Options > press the "Change" button (marked in Yellow) >

Select the version you want to use:

OK > OK > Apply > Restart R