How do I retrieve a text message accidentally deleted?

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

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

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

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