If you are staying with your current provider, it should be relatively easy to change to a postpaid plan. Your provider can help you choose a postpaid plan that meets your needs and budget and you can typically switch from a prepaid plan to a postpaid plan online, by phone or in person. You may need to provide your personal information and credit history.
If you are switching to a new provider, you will need to contact them and let them know that you would like to switch. You may also need to provide them with your personal information and credit history. Once your new service is activated, your old plan will be cancelled.