How do I restore deleted messages

In iOS 16 you can restore deleted messages from a Recently Deleted store, which holds them "within the last 30 to 40 days" as mentioned in Apple's support page Delete and recover messages on your iPhone or iPad.

If you do not have iOS 16 - and if your iPhone is as you indicate a 6s Plus you it can't support iOS 16 - or the messages you want were deleted more than 40 days ago you cannot retrieve them.