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:
Delete everything in folders that was shown after running .libPaths() in R. In my case, it looks like:
“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:
Various files and folders under C:\Users\%USERNAME%\AppData\Local\Temp related with R and RStudio sessions.
Then you can move on the re-installation side for R (after suggested reboot of Windows) as below.