You will see a list of people you have previously blocked. Find the name of the person you want to unblock and tap the button next to their name.
If you happen to come across the blocked person’s profile (via a mutual friend’s tag, old message, or search), you can unblock them directly: how do you unblock on facebook
Note: If you can’t find their profile by searching, use the main Blocking list method above. You will see a list of people you have previously blocked