Biden isn’t the “official nominee” yet but under DNC rules, the pledged delegates are essentially required to vote for the candidate their state selected in the first round. Only if no one has the required 1,968 pledged delegates in the first round does it go to a second round. At that point, 739 Superdelegates — party officials, basically — are allowed to vote and pledged delegates can switch to another candidate.
Biden currently has 3,894 pledged delegates. Second is Dean Phillips with 4. So, you’d basically have to convince thousands of Biden loyalists to change the party rules so they could vote for another candidate. And then, in the potential second round, convince the Superdelegates to also reject Biden.
That won’t happen unless he’s literally dead or incapacitated. Both primaries are over and the nominating conventions are a formality. It’d probably be easier to convince the electoral college to vote against the candidate that specifically chose them for their loyalty.