How do I change the PGP key with the new e-mail encryption settings?

How do I change the PGP key with the new e-mail encryption settings? E.g. if at some point I lost the old one or for some other reason wanted to use a new one for PGP at rest?

I don't really want to use an addon, I don't really understand the new option. When I activate it, it doesn't show me any form to enter a key either. I just can't read my mail using the webmail. I'm missing the old form where you could just paste a key.