If you mean for future backups, you would edit the backup spec Options tab.
Protection can also be set in the schedule tab, e.g. Highlight a date, Add, and here
a "Backup protection" value can also be set.
The protection in the Schedule, if set, overrides the protection in the Options tab!
Perhaps you mean you want to reduce protection from backups that have already run.
In the IDB context, left pane, highlight the backup, right click, and there is "Change Data Protection".
A common problem is that people want DP to use a tape, but DP won't use the tape because
one backup on the tape is still protected. If this is the case if you want DP to use the tape, Recycle the tape.
Recycling will take Protection off all backups written to the tape and DP can then overwrite the data.
Of course you loose the data that was backed up to that tape!
I just saw your point "How i can change a media protection without losing data?".
If you want DP to protect your data you should make sure the data remains protected.
If you change protection from Permanent to, for example, 30 days, you have to understand
that after 30 days DP may overwrite the data!
Hope this helps.