Fresh install Big Sur without deleting a partition Can I create a partition and store files I want to keep there, then do a fresh install to upgrade to Big Sur, or will the fresh install delete all partitions?
While upgrade procedures are tested before macOS is publicly available there always remains a slight risk that things go wrong and data gets lost. To be on the safe side, always make a backup on an at least one external device (two devices are even better) before upgrading macOS.