How to get back deleted text

If the backup isn't in iCloud, you'll need access to a computer with iTunes. Your backup must be from before the text were deleted. Note that any data on the iPhone from after the backup was made will be permanently lost.

First, you'll have to erase your iPhone - How to factory reset your iPhone, iPad, or iPod touch - Apple Support

Then, you can use either a Mac, or a PC with iTunes downloaded to restore your iPhone -

Restore from an iCloud backup

Restore from a backup on your computer with iTunes

Hope that helped!

Delete a message or conversation

When you delete a message or conversation, you can't get it back. So make sure that you save important information from your messages.

You can, but it requires a full restore from an iTunes backup. You can't just choose to transfer messages.