Use your iPhone, iPad, or iPod touch with iOS 10.3 or later
- Go to Settings > [your name] > Name, Phone Numbers, Email.
- Next to Reachable At, tap Edit, then delete your current Apple ID.
- Tap Continue. Enter the Apple ID that you want to use.
- Tap Next.
Use your Apple ID account page
- Go to appleid.apple.com and sign in.
- In the Account section, click Edit.
- Under your Apple ID, click Change Apple ID. If you try to change your Apple ID to an @icloud.com email address created within the last 30 days, you might be asked to try again later.
- Enter the Apple ID that you want to use.
- Click Continue.
Full article at https://support.apple.com/en-us/HT202667#otheraddress.