I still have my iPhone but I changed my number. I want to use IMessage with my new number. How can I do that?

You have to get Messages and FaceTime to “forget” the old number. These steps should achieve that:

1. Go to Settings/Phone and change the number to the correct one there.
2. Go to Settings/Messages, turn it on if it is off, tap on Send & Receive, then tap on your Apple ID and sign out.
3. Go to Settings/FaceTime, turn it on if it is off, then tap on your Apple ID and sign out.
4. Restart your phone using this “how to” link —>Restart your iPhone - Apple Support.
5. Go to Settings/Messages and sign back in.
6. Go to Settings/FaceTime and sign back in.