HC, I was just about to disagree, but given that you are never wrong, I figured I'd best explore every conceivable meaning of "Just click on it again." I swear it took this dimwit five minutes to figure out what you meant. 

To misquote a recent American president, "It depends what 'it' is."

So to be precise, to remove a previous "reaction" of "like" or "funny" or "disagree," or whatever, you click again first on the "React" button, and then click again on "like" or "funny" or "disagree" or whatever reaction you wish to remove.