Update
My MacBook has been stuck updating for 3 days and nothing is working. Then I followed the recovery mode thing and I don’t have enough storage to install the MacOS thing. What do I do??
This suggestion will involve another computer connect to the malfunction computer. The method is call Target Disk Mode. If successful, one makes able to transfer enough personal files, photo, videos, music off the problem computer to the other computer and thus create enough space on the problem computer to allow operating normally. The one will have to do a very good removal of unneeded Applications etc to allow upGrading Big Sur