Hello jacquiepu
The best way to do that is to go on your iOS device and go to Settings > iCloud > Storage & Backup > Change Storage Plan. The best way to know is that if you see the button that says Purchase Current Plan, then that means that you have not purchased anything and it will revert to the free 5GB plan.
iCloud: Complimentary storage plans expire on September 30, 2013
http://support.apple.com/kb/HT5527

Thanks for using Apple Support Communities.
Regards,
-Norm G.