By mistake I gave a thumbs down to an answer, does anyone know how I can reverse this?

No, it happens sometimes, don't worry too much about it. They don't really have any impact on points.

If you want to assuage your guilt, you can click on their profile and give then two thumbs up for some of their other answers. (One to reverse the thumbs down, and one for the original one you wanted to give.)

I learned like yourself that you can't take back the thumbs down.

I think you can't. Happened to me, too.

You can't

You can't.

unfortunately no