How can I change an existing type from "bigint" to "bigserial"?

I have a PostgreSQL table with the following structure: I simply need to change the TYPE of prove_identity_id from bigint to bigserial. I read the docs but wasn't able to understand how to legally achieve the change without the following error: PostgreSQL said: type "bigserial" does not exist PostgreSQL 10.1 on x86_64-apple-darwin14.5.0, compiled by Apple LLVM version 7.0.0 (clang-700.1.76), 64-bit