Q: How to downgrade iCloud storage?

See the instructions below from iCloud storage upgrades and downgrades - Apple Support

You can decrease the amount of storage by selecting a new plan right from your iPhone, iPad, iPod, Mac, or PC (for example, a 50 GB plan to replace a 200 GB plan). The downgrade will take effect after your current monthly or annual subscription ends.

iPhone, iPad, or iPod touch:

1. If you're using iOS 8 or later, go to Settings > iCloud > Storage. If you're using an earlier version of iOS, go to Settings > iCloud > Storage & Backup.
2. Tap Change Storage Plan.
3. Tap Downgrade Options and enter your Apple ID password.
4. Choose a different plan to use when your current storage plan expires.
5. Tap Done.

Mac:

1. Choose Apple menu > System Preferences, then click iCloud.
2. Click Manage in the lower-right corner.
3. Click Change Storage Plan in the upper-right corner.
4. Click Downgrade Options.
5. Enter your Apple ID password, then click Manage.
6. Choose a different plan to use when your current storage plan expires.
7. Click Done.

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 to use when your current storage plan expires.
7. Click Done.

If you cancel your plan or you don't renew it, you'll automatically go back to 5 GB of free storage.

I am having the same issue.  I can change to free but the done option is grayed out, both on my iPhone and on my PC.  it is not working?