I more or less follow this guideline:

1. Run installr::updater() from ../R-4.0.0/bin/x64/Rgui.exe (as suggested if you run it from RStudio)
2. Update all packages (check the "update all packages" box from installr::updater())
3. Change Windows system environment variable R_LIBS in my case to ..RR-4.0.xlibrary.

If I want to switch the R vesion from RStudio, I need to

1. Set Tools -> Global Options -> General -> R version AND
2. change R_LIBS to the correct place. (If you forget this, .libPaths() has more than one entry.)

So my questions are:

• Is that process correct? Do I miss anything?
• What about changing Rtools when switching between major R-versions, see e.g. here?
• It seems installr::updater() does not update the packages, although I check the "update all packages" box. (We observed that behaviour on two different laptops.)

Background: I had this question which was reproducible before I upgraded R to 4.0.2, but I realized, that after switching between
R 3.5.1, 4.0.0 and 4.0.2 back and forth for testing it is not reproducible anymore.
Thus I think, that my update-process was error-prone.

Source: Windows Questions