Down grade: Where can I download past stable firmware?

My 6pro solar  APEC can not pair with phone (Display: initiate) and bluetooth headsets after upgrade.

I did some research find that if I delete all maps, the function could restore  for minuets, but fail soon.

I think it is a software problem, so where can I download past firmware? I'd tied the Beta-area, does not work and becuse APEC version, I can not join beta program either.

Any suggestion?