Delete old backups in Settings > [your name] > iCloud > Manage Account Storage > Backups
Remove large files from iCloud Drive in the Files app or iCloud Drive on a computer
Delete unnecessary photos and videos from Photos
Empty the Recently Deleted album in Photos
Turn off iCloud Photos for devices you do not want syncing
Delete duplicate or unwanted messages and attachments in Messages
Remove old email attachments and large emails from iCloud Mail
Review and delete app data stored in iCloud in Manage Account Storage
Delete unused device backups
Clear shared albums you no longer need
Move files to another cloud service or local storage
Check which apps are using the most iCloud storage
Upgrade your iCloud storage plan if needed