If you carelessly give a thumbs down when you wanted to give thumbs up can you change it?

No that option isn't available,

Sadly, no. It's one of several things here that can't be changed when they're done - thumbs up and votes can't be changed either. It's one reason people shouldn't get too bent out of shape about 't.d.'s - you don't even know it was intentional. Fortunately, thumbs down doesn't harm anyone in any way.

If the person allows email or IM, you can contact them to explain if you like.

No.