Apple Watch Series 7 ringtone

That is currently not an option. This is a user to user support forum and Apple does not participate here. If you wish to provide feedback to Apple regarding a change you would like to see, a change you didn’t like, or report a bug, then use the feedback link here, Feedback - iPhone - Apple

You're welcome. If it helps, there never has been a way to select a different or custom ringtone on the Apple Watch, and we are currently on Series 7, so it would be out of character if it happened now.