How do I get back deleted messages ?

For messages, if you were syncing Messages with iCloud and deleted them, they can't be recovered. If you weren't syncing Messages with iCloud, do you have a backup you can restore from? Restoring from a backup erases all data currently on your device. Any data newer than when the backup was made will be erased. A restore is an all or nothing restore. You can't access a backup and only restore certain files.

Backup - Restore your device from an iCloud or iTunes backup. 

Restore from your last iCloud backup if available. If it's a photo, there's a "deleted" album that stores deleted photos for ~40 days.

Restoring from iCloud

1. Open Settings -> Tap on your photo at the top -> iCloud -> Manage Storage -> Backups.
2. Find your device.... if it's there (and was recently backed up), then you can proceed.
3. Go to Settings -> General -> Reset -> Erase All Content and Settings. NOTE: This will delete everything after the backup so make sure you save anything that's new.
4. Once completed and your iphone restarts, you can restore from iCloud Backup.