To update or correct your phone number for iMessage, go to your iPhone's Settings, scroll down to Messages, and tap on Send & Receive. In this menu, if your number is incorrect or missing, sign out of your Apple ID by tapping on the email address and selecting Sign Out. Afterward, restart your iPhone. Once it has restarted, navigate back to Settings, then Messages, and Send & Receive. Sign in with your Apple ID, and your correct phone number should now appear.