Hey flusteredparent,
Welcome to the Apple Support Communities! I understand that you are experiencing issues while trying to downgrade your iCloud storage plan. Do you have the iCloud for Windows application on your PC? If so, you can follow these steps to edit your storage plan:
iCloud storage upgrades and downgrades - Apple Support
https://support.apple.com/en-qa/HT201318
Windows:
- Open iCloud for Windows or iCloud Control Panel.
- Click Storage if you're using iCloud for Windows. If you're using an earlier version of iCloud Control Panel, click Manage.
- Click Change Storage Plan.
- Click Downgrade Options.
- Enter your Apple ID password, then click Manage.
- Choose a different plan to use when your current storage plan expires.
- Click Done.
If you do not have the iCloud for Windows application or an iOS device available, please contact us for assistance in downgrading your plan:
Get help using iCloud storage - Apple Support
https://support.apple.com/en-us/HT203527
[...] If you still can't upgrade your storage, contact iCloud Support.
Thanks,
Matt M.