It's hard to believe but it seems that nowhere on the Mathworks site one can find information on what is the best way to remove previous versions. There is one question from 2013 that still is unanswered.
I think it should be part of the installation instructions anyway.
Specifically I'm looking for instructions for an OS X machine. If it's only to delete the MATLAB_R2018a.app that still sits in the programs folder that would be also worth spelling out.