How do I delete all older backups from all devices associated with my account

If I go to Settings > [username] > iCloud > Manage Storage > Backups on my iPhone (as an example) I can delete backups of any of my devices.

Also note the following from Manage your iCloud storage - Apple Support

(it also has instructions for doing it from a Mac or PC; Bold added by me)

Delete backups and turn off iCloud Backup for your device

You can delete old iCloud backups for the device you're currently using, or older devices you may not use anymore. 

iPhone, iPad, and iPod touch

1. Go to Settings > [your name], then tap iCloud.
2. Tap Manage Storage > Backups. 
3. Tap the name of the device whose backup you'd like to delete. 
4. Tap Delete Backup > Turn Off & Delete.

 When you delete a backup, it also turns off iCloud Backup for your iPhone, iPad, or iPod touch.

See information in Manage your iCloud storage - Apple Support