How can I delete a collection?

I can’t find the delete button for collection of questions. That’s very strange. Can anybody explain it?

Collections can be archived by clicking the archive button in the top-right of the page while inside that collection (it’s the box with the downward pointing arrow):

Thanks for response. I noticed that. But if I added some collections for test purpose. I don’t need them at all. Why can’t we just provide a deletion function?

Yep, this is up for discussion, and there's an open issue for a related topic here (this issue is about archived questions rather than collections, but same idea):