Icloud plan switch

Welcome to the Apple Community. 

You can change the amount of storage by selecting a new plan right from your iPhone, iPad, iPod, Mac, or PC (for example, a 5 GB plan to replace a 200 GB plan). The downgrade will take effect after your current monthly or annual subscription ends.

iOS

1. Go to Settings ➔ Apple ID(Top) ➔ iCloud ➔ Storage ➔ Manage Storage.  
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.

OS X

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.

Windows

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