1$autorestic forget [-l, --location] [-a, --all] [--dry-run] [--prune]
This will prune and remove old data form the backends according to the keep policy you have specified for the location.
--dry-run flag will do a dry run showing what would have been deleted, but won't touch the actual data.
--prune flag will also prune the data. This is a costly operation that can take longer, however it will free up the actual space.