How to downgrade iCloud storage

On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage.
2. Tap Change Storage Plan.
3. Tap Downgrade Options and enter your Apple ID password.
4. Choose a different plan.
5. Tap Done.

See this:

https://support.apple.com/en-us/HT207594