How do I retrieve old messages

The only way to recover deleted messages would be to restore your iPhone with a saved backup. This process is time-consuming, and it would be dependant on you having a backup that has these text messages saved, so if you don't back up your phone regularly (I personally have mine done automatically which you can learn about here) then this process might not work.

You may also find this link helpful in regards to restoring your device from a backup.

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

If you were syncing Messages with iCloud and deleted them, they can't be recovered.