[Chimera-users] modeling selected protein mutations to a sequence

Hi Chimera users, I am trying chimera for the first time and my question is: - How can I change (substitute) specific amino acids within an uploaded protein sequence to mimic a missense mutation? - Or how can I enter a whole new custom made protein sequence into chimera, so I can probe different variants of a protein sequence. I don't know how to make the file like pdb (protein database) so I can fetch my custom sequence? Thanks! Teresa