Red button with trash can on it will delete the client from the list. Removed clients are just moved to the trash can from where they can be restored later on if you happen to need to use that client ever again. To delete a client permanently, open up the Trashed items list and delete it once more from there.
Clicking on the blue button will open up the client editor, where information of existing clients (the ones that you selected), can be edited. Layout of the editor is pretty much the same to the one that we described when talking about how to add new clients, in fact it's exactly the same to that one.