How do I retrieve my messages from iCloud backup

You cannot extract Messages from a backup. You'd need to restore the entire backup.

If you go to Settings > [your name] > iCloud, is the switch turned on for Messages (on either iPhone?)

If that switch is on, then Messages are not included in your backup anyways.

If that switch is turned on for the iPhone 6s, but not the iPhone 7.. Then you should simply need to turn it on for the iPhone 7 and the messages would in theory start loading.

If that switch is not turned on for the iPhone 6s, I would do a backup to a computer myself.. (piece of mind as they are important messages.. even archive the backup for that matter)

Then turn on the switch for Messages in iCloud on the iPhone 6s. Give it a bit to upload to iCloud.. turn the switch on for the iPhone 7.

The following questions are all for your iPhone 6s.

1. Do you use iCloud for Messages? Open Settings ➔ Apple ID (Your Name at Top) ➔ iCloud ➔ Messages: Is the switch for Messages ON or OFF on your iPhone?
2. Do you back up your iPhone on iCloud?
3. Do you back up your iPhone on a computer?

Axel F.