How to pause, cancel or delete a scheduled CiviMail mailing
If you search for mailings (Search>Find Mailings) and check 'Scheduled' when results com up on the far right there is an option to 'Cancel' under the 'more' menu.

I assume you can pause by searching for 'running' mailings and looking under the 'more' menu but I can't verify as I don't have a running mailing right now.