A lot of what needs to be done is making sure that the Harris win is large enough that you can’t easily claim that a handful of ballots should be tossed and change the outcome. That means:
- Check your voter registration — part of the Republican strategy has long been invalidating registrations so people can’t vote
- Volunteer — nothing in the world quite like talking to people.
- Donate — money is used for everything from ads to voter turnout operations
- Organize; be prepared to turn out with others in your community to actively object to any effort to ignore your votes
So the term for this is “faithless elector” and the answer is, it depends.
For some states if an elector promises to vote for X, and then they vote for Y, they are immediately replaced and their vote does not count. In other states the vote stands as is.
Most commonly a faithless elector isn’t used to vote for one of the major parties, but for a third party or someone who didn’t run for president. For example if Trump won a given state, but an elector refused to vote for Trump, they will likely vote for another Republican.