Delete master pages not working, load new master from file

Hello,

I exported a powerpoint file to odp.
Now I want to edit the master layouts of this odp presentation and adapt it to new needs.

Unfortunately, I can not delete any of the master layouts, how can I solve this?

Furthermore I have created a new master layout and saved it as a otp file.
How can I load this file to replace the layout of my existing presentation?

I am using version 7.1.6.2.

Thanks and kind regards
Andi