If you want to affect the outcome of the election, you will need to change a lot more than a single vote. Each ballot you steal, next to worthless all by itself, opens you up to detection, and vote tampering comes with heavy penalties.

You need to find a systemic vulnerability, otherwise the risk is much too high and the odds of success much too low. Vote-by-mail isn't actually a worry unless you have an ideological position based on the current election, you should really worry about the electronic voting machines used in many places that have in-person voting. The potential for systemic fraud there is much, much higher.