If you want to change or alter your additional iCloud storage what you will need to do is:
1. Go to Settings on your device (iPhone, iPad, iPod Touch)
2. Tap your name at the top of settings (iOS 11 or later)
3. Tap iCloud
4. Tap manage storage
You can then change, upgrade, or cancel your plan at any time. If you are having issues with canceling you can also call AppleCare at 1-800-275-2273 for assistance.