How do I view deleted messages

There is no way to view something that is not there. They have been deleted. They are gone.

You can erase your device and restore it to a backup that was made before the messages were deleted, but remember, if you restore your phone to an backup, your phone will revert to having the data that was on the phone at the time of the backup. Anything that has been added, deleted, or changed since the backup would be overlaid by the previous data:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Cheers,

GB