How can I cancel the icloud upgrade i purchased today?

This is the link for iCloud Support:

https://www.apple.com/support/icloud/contact/

However they do say - in a comment added only recently -

'iCloud support is free during your Apple hardware product's complimentary support period or with an AppleCare Protection Plan. Depending on your hardware product's coverage, you may need to pay for support. iCloud support is currently available by phone.'

The implication is that if you aren't in the AppleCare window you can't get any support - which seems unreasonable if there is a payment issue. You can only try the contact method and see if you can get help - get your hardware number from 'About this Mac' before you start.