HI.
When i "edit" the schedule for my backup rules, i can only change how often and when it should run, but i cannot change how much to keep, right now i ave 790GB of backups of full and incremendal/Differential backups. I changed the defaults, but the backup job still has its old values

It sounds like you're editing the schedule instead of the retention rules.  You access both from the same page.  If you want to change the values of a specific existing definition file, you must select that file and then edit it.
The defaults only affect definition files going forward. They do not change definitions that already exist.

NOTE:  Before attempting please wait for confirmation from other forum members.

I am Editing it, and this is only what i get to change:

Have you tried looking for "My Backup.xml" under "Backup Definition Files", and editing it from there?

                                

@Gargamel You are still editing schedule entries, and that's not where retention rules are maintained.   They are in the XML definition file.

From the Definition Files tab of the Create Backups display, select the file My Backup.xml,and press the Edit icon.
The options for retention rules are changeable when you press Next to get to the second page of the display.   Be sure to click finish if you changed anything.

@DanDanz  OP is using v7, so I think the "Backup Definition Files" tab in the "Backup" view is the appropriate place to look.

                                

^ Correct. @Gargamel, don’t use the Scheduled Backups tab. Use the Backup Definition Files tab. That’s where you edit the entire job, as opposed to a particular schedule within a job.