Q: i purchased additional icloud storage and don't need it; how can I cancel or downgrade?

See this help page:  iCloud storage upgrades and downgrades - Apple Support

https://support.apple.com/en-us/HT201318

Downgrade your iCloud storage plan

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.

If you downgrade your storage plan and your content exceeds the storage you have available, new photos and videos won't upload to iCloud Photo Library and your devices will stop backing up to iCloud. iCloud Drive and iCloud-enabled apps won’t update across your devices, and you won't be able to send or receive messages with your iCloud email address.

To use iCloud after exceeding your available storage, remove backups, photos, documents, or email messages you don’t need. You can access photos, videos, and documents for 30 days after you delete or disable iCloud Photo Library and iCloud Drive.

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.