How do I cancel iCloud payment (or downgrade) without an i device?

If it's been more then 15 days since you ordered the storage, then you can't get a refund.

otherwise, use a computer browser...

For customer service issues, go here...

https://expresslane.apple.com/Issues.action

You can't do this from the iCloud website, you will need a Mac or an iOS device with the iCloud preference pane. If it's more than 15 days since you purchased the storage you can only cancel it at its next renewal date, so you have until then to do it. The method is explained at

http://help.apple.com/icloud/#mm12eecb08e7

If you are not going to be able to do that, or if you are still in the 15 day period and want a refund, you may be able to do this using http://expresslane.apple.com - Click on 'More products and services', then 'iCloud', 'Account and setup' then 'This topic is not listed'. Explain the situation in the box provided and wait for a response - include a phone number. However you may find you cannot proceed without entering a hardware serial number which is still eligible for AppleCare.

In that event you will need to phone them directly, using the Apple Online Store number given at the bottom of this page. The first person you speak to may tell you to do it from the iCloud website but this is not possible. Politely insist on escalating the issue to a more senior person and eventually you should be able to get through to someone who can deal with this (you may have to wait for them to call you back) - you will be asked security questions and for part of your credit card number to identify yourself.