How can I retrieve accidentally deleted iPhone text messages?

If you're using iCloud Syncing for Messages, once you delete a message, it's gone.

If you are not using iCloud Syncing and you have a back up that contains the messages, you can restore your phone from that back up.

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

Note that restoring your phone from a back up will overwrite everything currently on the device.