System Memory OS 10.12.6 Sierra

Having to use the old MacBook Air while the 2020 MBP has board replaced due to port failures. I'm having the old issue of system storage using 105 GB of the 128 SDD. I am not a tech expert & never could get this squared away back when it first happened ages ago. Again I'm back on this Air because of a hardware failure with the MBP.

I've tried several things through Google searches like investigating Time Machine local snap shot issues- keep getting Terminal error- Unrecognized Verb (for another time). I did find a mobile backup search that shows large Document Revisions (81G). My question is what to do next? I'm dependent on this laptop right now & can't afford to "Crash" it. Any suggestions would be appreciated!