Budget's website is notoriously bad in this regard. I would just call them to make the change. Just make the pickup and/or return time an hour earlier/later. Tell the agent that you want to confirm the new price before they complete the change. Once they quote you the new (hopefully lower) rate, just tell them you want to make sure there is no change fee. That's how I would approach it.

All of that said, before you do that, you might want to just get a quote using the link below. There's a chance we may be able to find you a significantly lower rate such that it may pay to just cancel and pay the $50 fee and re-book a non-prepaid rate (which could go down further). It's a bit of a long shot, but you have nothing to lose, and at least this way, you'll know what the bottom-line price is.