How do you update a skin?

The only way I found out, was to just re-add the updated skin. Transfer all the users from the old one to the new one, then delete the old one. Seems like a bit too much work to me.

Am I missing a way?

I do not think there is an easy easy way to update a skin, apart from deleting it and reuploading.

The problem with this method is that anyone who had that skin, (I assume) goes back to the default skin.

Remove and Re-Add, or get a differential between the two versions are the only ways as far as I know.

What's the reason for skin updating, if I may ask?

if the author releases an updated version. For example 'cleancut' here