icloud storage

If you mean manage your iCloud storage, see this.

Manage your iCloud storage - Apple Support

If you want to downgrade your iCloud+ storage plan, see this.

Downgrade or cancel your iCloud+ plan - Apple Support

It's not clear if you're referring to iCloud in general, or iCloud Drive.

At any rate, referring to the iCloud user guide may reveal the answer for you:

Delete files in iCloud Drive on iCloud.com - Apple Support