How can I get back deleted messages

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)

What does iCloud back up? - Apple Support

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

If you backed up your phone before you deleted the messages, then you can restore the phone to the time of the backup and view the messages.

For example, if you backed up your phone on May 23......when the messages were still on the phone......you can retrieve the messages when you restore the phone using the May 23 backup.

Unfortunately, if you do this, you will lose any new data that you have entered on the iPhone since May 23. In other words, you phone will look like it did on May 23.

Bottom line......best to not delete a message unless you are sure that you will never need it again.

You can only recover deleted messages if they were included in a recent back-up of your phone.

When you delete a message pay attention to the warning which says ‘this action cannot be undone’ and you have your answer.