How do I delete an iCloud backup?
Tap Settings and then tap the iCloud icon.
Tap "Storage &amp; Backup."
Tap Manage Storage.
Tap your iOS device in the Backups area.
Under Backup Options, turn off any items that you no longer wish to back up.
Return to Storage &amp; Backup and tap “Back up now.” This will immediately remove the requested backup data from iCloud.
Choose “Delete Backup” if you want to turn off Backup and remove all data for that Backup from iCloud.


(via support.apple.com)