I\'m on osx6.8 and need to install an earlier version of Mongodb, how do I install an earlier version with HomeBrew? The below didn\'t work :(
dream-2:app2
Addition to the excellent answer of joeytwiddle :
if you don't want to link then unlink the old version of the software, you can just run it from the "cellar" (/usr/local/Cellar/), where brew installed it.
Use ls /usr/local/Cellar/ to find the exact path of the executable. For example in my case, to run mongo shell:
/usr/local/Cellar/mongodb@3.6/3.6.7/bin/mongo