How do I delete old save games?

OK. It is NOT in the manual because it CANNOT be done.

What can be done is to backspace over the existing 'save name'
forward space at least one space
type in a new save name.
add an extra space
hit enter

SOLVED Sort of...

Dude, instead of creating three different threads:

(this one),
https://www.gog.com/forum/general/dungeon_hack_cleric
and
https://www.gog.com/forum/general/dungeon_hack

edit your posts.

Even better yet: post your game related questions in the dedicated subforum:
https://www.gog.com/forum/forgotten_realms_collection#1682888037
or (Dungeon Hack related):
https://www.gog.com/forum/forgotten_realms_collection?search=hack

And finally: you are aware that there is a manual included in your download files?
Why not have a look into that?

You have to delete them directly from the game's file folder if I'm not mistaken. At least I think that's how I did it once.

Nah, I'm gonna tell you what, instead.
If you are unable to delete saves manually, maybe you shouldn't use computers.
And one more thing, while I'm at it: even if the manual had instructions on how to delete save games - I wouldn't do the work for you.
You want to know how stuff works? Look it up.