How do you remove a reaction made to a conversation or comment?

You can make up to 5 reactions on any conversation or comment. But to reset the reactions you have made is easy, just click the React icon, but before you select one scroll the options you see all the way to the right end, and you will find a "Clear Reactions" button.

You can also click the 3 dots on the right side of the conversation or comment and in that menu you will see the option to remove reactions.