Once I have tried to uninstall R related sides, I realised that it is not straightforward unlike other programming engines. For that reason, I have posted the procedure on the stackoverflow website under the related questions. 

I have further issues with the answers on the forums/internet. For that reason, I have extended the solution with further steps (which I experienced currently) as below:

1. Uninstall R, RStudio and RTools from Windows “Programs and Features” menu.

2. Delete everything in folders that was shown after running .libPaths() in R. In my case, it looks like:

   .libPaths()

   “C:/Users/%USERNAME%/Documents/R/win-library/3.6” “C:/Program Files/R/R-3.6.3/library”

Therefore, I have manually deleted following files and folders (they remain there after full uninstallation of R, RStudio and RTools) as well:

• C:\Users\%USERNAME%\Documents\R\
• C:\Users\%USERNAME%\Documents\.Rhistory
• Various files and folders under C:\Users\%USERNAME%\AppData\Local\Temp related with R and RStudio sessions.

3. Delete everything in C:\Users\%USERNAME%\AppData\Local\RStudio-Desktop
4. Delete everything in C:\Users\%USERNAME%\AppData\Local\rstudio

Then you can move on the re-installation side for R (after suggested reboot of Windows) as below.

In order (please): Download R for Windows and RStudio Desktop. (If you need, download also RTools)

• Install R
• Install RTools (if you need)
• Install RStudio.