I would just leave it alone. Add your flight info (if you're flying in). Generally, they'll hold a reservation for at least 6 hours (often much longer), so I wouldn't worry about changing it if you'll be arriving within a few hours of your originally scheduled time.

Likewise, no problem returning a day early. You're almost certainly paying the weekly rate, so your base rate probably won't change, but you'll probably pay one less day in taxes/fees for the ones that are levied on a per day basis.