how do i downgrade icloud account

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

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.