In order to upgrade R to a new version, it’s best to first uninstall the old version of R, and then install the new version.
Here is how to completely uninstall R on Mac, if R has been installed from the R website:
rm -rf /Applications/R.app
sudo rm -rf /Library/Frameworks/R.framework
sudo rm /usr/bin/{R,Rscript}
Note that user-installed packages from the old version will be lost and have to be reinstalled in the new version.