You would have to Erase your device and restore it to the iCloud backup that had the messages still on it. If you want to do that, you need to go to Settings>General>Reset>Erase All Content & Settings. Then it will prompt you through setting up the phone like it is a new device. When you get to the part where you have the option to Restore from Backup, select the iCloud backup you want to use.
Remember, restoring to the backup will put your device back to where it was exactly at the time the backup was made, so anything that has been added or changed since then will revert back.
Also, before doing this, be sure that you have a backup, and it would be a really good idea to also back up your device to iTunes just to have a second backup option.
Best of luck,
GB