I'm planning to falsify the expense report tonight.
Are you really going to falsify them?
It's for revenge on my boss who has been so mean to me recently.
You shouldn't falsify anything, because that can be considered illegal. You're not doing a prank but instead committing a felony and could go to jail if you're caught lying against your employer.
Fine, so I should always do things the hard way?
You should not mess with stuff like that, theft and tax evasion are federal crimes. You don't want to end up with a felony. You can report your boss to HR if you feel you are being treated unfairly.