how can i get deleted messages back

go to Settings > [your name] > iCloud and check the switch position for Messages

If the switch is ON, then you cannot recover your deleted messages and they are not contained in an iPhone backup

(no need to read any further)

If the switch is OFF, then you might be able to recover your messages IF you have an iPhone backup that contains the messages you are trying to recover. In which case you'd need to restore your iPhone to that backup. In doing so, you'd likely lose some data between now and whenever your backup occurred.

Locate backups of your iPhone, iPad, and iPod touch - Apple Support

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

Chris0107 has already provided instructions for using iCloud to try to restore Messages.

IF......you have been backing up your iPhone to your Mac......AND......you have been backing up your Mac to another hard drive using Time Machine.......you might be able to retrieve messages that way. It's complicated.

No point in going into more detail here, unless the conditions above have been met. Post back if that might be the case.

As in the iCloud method to restore things, the Time Machine backup method works in the same way. You will be restoring your iPhone from a backup from the past, so you will lose any data that has been added to the iPhone since the time of the backup.