Delete old backups in Settings > [your name] > iCloud > Manage Account Storage > Backups
Turn off iCloud backup for apps you do not need
Delete photos and videos from iCloud Photos
Empty the Recently Deleted album in Photos
Remove large files from iCloud Drive
Delete old files from the Files app in iCloud Drive
Remove email attachments and old messages from iCloud Mail if using iCloud Mail
Delete old voice memos stored in iCloud
Remove unused device backups
Review and delete app data stored in iCloud
Turn off iCloud syncing for apps you do not want to store in iCloud
Upgrade to a larger iCloud storage plan if needed