If you already voted for a presidential candidate who has now dropped out of the race and the primary election still hasn't taken place (e.g. Super Tuesday), in some places you can apply for a new ballot. Call your county clerk to check it out.
One of the downsides of so many voters with disabilities having to use absentee ballots due to lack of access in voting places are situations like this. It's time to make voting accessible so our votes count equally.