The address information you see is taken from your GMRS license. When you update your address with the FCC, your address on myGMRS.com will typically update in 24-48 hours from the date the FCC processes the change.
Please note that you will need to update the address on the GMRS license in particular, not just on your FRN account with the FCC, which is separate and not visible to us.
Remember, just like with new callsigns, it can take a day or two for the FCC to notify us of the change.
Your username is typically your FRN number, which you can find by looking up your FCC license.