Finding GRASS module after upgrading QGIS (Mac)?

I have figured this out and thought it best to post the answer for future users.

Firstly, I right clicked on my QGIS application and selected 'Show Package' to show the subfolders within the application (apologies if this is obvious to most users but it wasn't to me!)

In doing this I managed to locate the exact path of my GRASS installation, which is /Applications/QGIS.app/Contents/MacOS/grass7.

I replaced both GRASS folder locations shown in the screengrab above (within my original question) with the correct path, but only ticked 'Activate' on GRASS GIS 7 commands. (As I assume these supersede the standard GRASS commands.)

This still didn't fix the problem, however I then noticed in Plugins > GRASS > Grass Options, under the General tab there is an option to insert a custom file path to the Grass installation, so I pasted the same file path in here.

After restarting QGIS I now have working GRASS again.

I had the same issue. There was an error with the path. I have QGIS 3.14, and a Mac running on Mac OS 10.15.4 Catalina. The path for grass was /Applications/QGIS3.12.app/Contents/Resources/grass7.

I went in the Settings >> Options >> Processing >> Providers >> Grass and I wrote the new path with QGIS 3.14 instead of 3.12. Wrote the same path in the grass plugin (Plugin >> Grass >> Grass Options >> General >> Custom)