Could be tricky...
If Joe Biden were to withdraw at any point before the Democratic National Convention, it would be up to the delegates at the DNC to pick a new nominee. But from there, they can expect legal challenges from Republicans looking to prevent any other Democratic nominee from getting on the ballot in certain states.
Wisconsin does not allow withdrawal from the ballot for any reason besides death.
In Nevada, no changes can be made to the ballot after 5 p.m. on the fourth Friday in June of an election year or 'a nominee dies or is adjudicated insane or mentally incompetent.'
If Biden were to withdraw less than 60 days before the election Georgia his name will remain on the ballot but no votes will be counted.