Delete back up on time machine
How do you delete back ups on time capsule to make space
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Best way by far is to use the airport utility and erase the TC.. quick erase is fine and it will take you back to empty disk in 2min flat.
Then do backups starting with any computers you have running big sur. If none that is fine use the latest OS you have and as each completes start the next one.
If you wish to keep all the old backups except one or two old ones.. you can mount the disk in finder and just just delete the sparse bundle. This will take a longish time.
You could also archive the TC disk to USB disk of equal or larger size. This tends to be my preferred way to do things. Once archived you can then erase the drive. You can see archive and erase below the big Partitions box on the disks tab in the Airport Utility.