I fought an opponent a while ago and I'm not sure how to quit the battle...

go to the Coliseum. At the top of the Challenger page, there's a row of links. Choose Statistics. Scroll down on that page until you find your battlepet and choose Forfeit.

thank you so much!

you are welcome. Some Challengers will filch all your weapons and some aren't strong enough to hurt you even though you yourself can't punch through their mighty turtle defense, so this is a thing battlers need to know.