How to cancel iCloud Storage Subscription without IOS, OSX or Windows device?
If you can borrow a computer or iOS device, you can use that to sign into iCloud and then change to settings that way. This is probably the only way to do it yourself without calling Apple directly and seeing if they will do it for you.

Apple devices will have iCloud software already installed, but for Windows machines, you would need to download the iCloud software and install it - https://support.apple.com/en-ca/HT204283

Other than installing a VM running Windows, there is no iCloud software for Linux.