Unfortunately, the chance of the nominee being anyone else is pretty slim, short of Biden having a sudden and severe medical issue. You can lodge a vote for one of his challengers in the primary, but it's little more than a protest vote - not that that should discourage you, but looking at it realistically. Most of the theoretical candidates being put forward to replace Biden have the very, very small issue of not actually running in this election, and that's kind of a pre-requisite to win an election.