What is the most appropriate opposite of “Select All”?

Select None would be so that (in software terms) that NO categories are to be selected at all. Unless you assign it a specific language code (for example) to make it do the same as Clear/Deselect All. especially for select none, which is common.

1. Is there such a word as unselect?

Is there such a word as unselect?

Verb. (computing, transitive) To cancel a previous selection, especially by removing a mark from a tick box. (transitive) To reverse the previous selection of.