i am unable to update to El capitan!! In app store it's always saying failed to download.. and to "use purchases page to try again"

Try resetting the App Store and Clearing Cookies.

Reset Learned from Old Toad

tried resetting and clearing cookies under the Debug Menu for App Store, but the problem still persist.

any other possible remedies? thanks in advance.

2012 11" MBA

Try downloading in the Safe Mode and/or create another admin account to see if you can download it there.

Safe Mode

Safe Mode - Mavericks