I want to install the latest release from source, and to avoid confusion I
want to remove all trace of the current installation (4.2.0-rc3) Is
rm -r /opt/octave-4.2.0
all that is required ?
This depends on how you installed the RC3. Assume you did a typical `./bootstrap && ./configure && make && make install` why not doing `make uninstall` from your build directory and removing the directory itself afterwards?