how do i change my address or apartment number

If you are talking about setting up a forwarding address after you move, then you can either fill out a change of address form at USPS.gov, or visit the post office and fill out the form there. But this only gives you 6 months of mail forwarding, and in that time period you need to update all of your bills and contacts and such with your new address.

Go to the US Post Office or online