how to I get back deletedmessages

If you have a device backup that contains the messages, and you do not use messages in iCloud, you can factory reset your iPhone and restore it using the backup. You'll lose anything new since the backup was made but you might recover your messages.

Still applies as above. If you use messages in iCloud, your deleted messages are irrecoverable. If you do not, and have a backup, you may be able to restore.