How do I delete old backup files?

Additionally you can prune the backup yourself. What I did is to use:

duplicity remove-all-but-n-full 1 --force scp://mylogin@myserver/path_to_backup

from command line. Just use the information you used to create your backups from the Ubuntu GUI.

You can use the Dconf app from Ubuntu Store to modify setting at path org.gnome.DejaDup, key name delete-after. It's set to the number of days to keep backup files on backup location.

Or from terminal. For example, to set it to 60 days from the command line, run:

gsettings set org.gnome.DejaDup delete-after 60

Deja-dup does not yet supply a way of removing old backups, you should also not delete some of the files, that will leave probably your backups without a start file and renders them invalid. Remove them all and start over is an option but thats not what you want I think.

Deja-dup keeps backups for the specified time or until the backup space is full, it will them manage your backups accordingly, a solution for your problem might just be enable quotas for the backup drive and don't let it take all the space available or change the backup frequency, once a week if you edit many files or are always copying / moving files will leave you with a very large backup image.

The answer of https://askubuntu.com/a/94288/676490 for a local backup:

duplicity remove-all-but-n-full 1 --force file:///home/username/deja-dup