cancel subscription

Cancel upgraded iCloud storage by downgrading to the free 5GB plan. See instructions in: Downgrade or cancel your iCloud storage plan - Apple Support

Also see excerpts below for doing it from a PC:

Downgrade or cancel on your Windows PC

1. Open iCloud for Windows.
2. Click Storage.
3. Click Change Storage Plan.
4. Click Downgrade Options.
5. Enter your Apple ID password, then click Manage.
6. Choose a different plan.
7. Click Done. Can't click Done?

If you don't have iCloud for Windows, also see: Set up and use iCloud for Windows - Apple Support

or

• If you don't have one of these devices, contact Apple Support to change your iCloud storage plan.