How do I unlike a photo?

If you’ve accidentally double tapped or tapped below a post, you can remove the like by tapping again.

Things to keep in mind:

Very soon after you remove a like, the like alert will disappear from other people's Activity.
If you've accidentally liked a photo and then unliked it, the person who posted it will still get a push notification if they have notifications turned on.
If you like, unlike and then re-like a photo, the person who posted will only receive one Activity alert for the latest like and one push notification for the initial like if they have push notifications turned on.