how to retrieve deleted text messages from a contact

If you have iCloud Messages (Settings > your name > iCloud > Messages = ON) then they are gone and you cannot retrieve them. If you did not, and the deleted messages are in your last backup, you can restore your iPhone from your backup. Keep in mind anything added since the time/date of that backup will be gone.

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 included 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

If you have automatic backup turned on and you don't have iCloud Messages on then yes they will be backed up to your iCloud in the backup file. But what you can get back when depend on when the messages came in, when you back up and when you deleted them.