If the version is no longer in the active repository, you may need to build it from the source tarball available on the CRAN "Source Code for Leaf Directories" page. Managing Multiple R Versions
Visit the CRAN Archive: Go to the CRAN Windows bin archive page. Select the Version: Look for the link labeled "R 3.6.3". download r version 3.6.3
Run the .exe: Open the downloaded file and follow the installation wizard. If the version is no longer in the