Are you second guessing your support of a candidate after filling in your ballot? Did you spill coffee on it? Perhaps your toddler used it as a coloring book.
There are plenty of reasons why you might need a new ballot this election. You can apply in writing for a replacement by filling out an application form and taking it to your county elections office.
You can also vote in person instead. You just have to bring your vote-by-mail ballot to your polling place on election day, and a poll worker will swap it for a new one for you to fill out there.