Deleting backup files older than the last one

General Help
1

How is that done?

2

Replace path/to/backups with the *actual filetree path to the backups:

duplicity remove-all-but-n-full 1 file:///path/to/backups

3

That's not a "Terminal" command is it?

4

Yes, that is a terminal command. While the vast majority of the time, a terminal does not have to be used, there are rare occasions when it is the only way.
As on MacOS or Windows OS, sometimes, though not often, you must use the prompt in order to make clear instructions to the system.

DejaDup does not have any built in GUI method of selecting that action, so the terminal is the only way to do it.