If you need to change your Apple ID to an email address that is not already an Apple ID, then you just need to follow these steps:
Change your Apple ID - Apple Support
Doing it any other way, you are likely to lose information that was tied to your previous Apple ID. For example, any apps that were downloaded with the previous Apple ID will no longer be able to update unless you provide the password to the Apple ID that was used to download. Also if you purchased any Music or Books with the previous ID, you will not have access to them with the new ID. If using iCloud Photos, Notes, or Messages in iCloud, you should have the option to save that data on your device when iCloud is shut off. If there is no iCloud data to save to your device, you will not get that message. The iCloud backup cannot be saved to your device.