Go to your collection and click EDIT. Click Show All. Scroll to the bottom and check "Select All" then use the scroll down menu and pick delete from collection.
a long time ago, shortly after first using the site, i tried the "show all" option under edit, and it reset all the quantity, box, and manual numbers in my entire collection folder to 0
. now i'm scared to touch that option, i never use it (show all that is). i still edit but only one page at a time.