You can go to budget.com and select Reservations > View/Modify/Cancel... enter either your original reservation number or your rental agreement number (with the latter, I had to add a letter U in front). Site will give you the new total cost (does work with reservations made via third party sites) and ask you to confirm. Includes a $10 change fee.

When I tried, however, clicking Confirm showed an error and stated that I needed to call. So I called the Budget location I picked up at and asked them to make the change and it matched with what the website had previewed.