How can I manually delete files and folders from a Time Machine backup volume?

We have 20 or so backups on a 3TB drive. We need to go into each backup and delete some specific folders from each.

Attempting to do so via Finder brings up "backup items cannot be modified". We tried to modify the permissions which took a long time but this didn't help.

macOS Catalina 10.15.7

Solution 1:

Golden rule: Never try to delete Time Machine from Finder.
If you're lucky it won't let you. If you're unlucky it will - & that's no end of trouble.

To delete the entire history of a file or folder, first select it in Finder then from the Time Machine menu bar item, select Enter Time Machine

You can then right click your unwanted item & select "Delete all Backups of…"

If the history goes back a long way, this may take quite some time. Be patient.