With the item, you can also make changes using an attribute list:
OSStatus SecKeychainItemModifyContent (SecKeychainItemRef itemRef, const SecKeychainAttributeList *attrList, UInt32 newPasswordLength, const void *newPassword)
The new attribute values would go into the attrList. A new password would go into newPassword.
To delete an item:
OSStatus SecKeychainItemDelete (SecKeychainItemRef itemRef)
Note that you would still have to call CFRelease() on the item to prevent a memory leak.