how do I delete old iphone backups

You have a few options, you can delete the backup from within iTunes itself. This can be done by going to Edit -> Preferences -> Devices tab -> Backups. Highlighting the backup you wish to delete and hitting the Remove/Delete button.

OR

On Windows XP:

C:\Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\

On Windows Vista or 7:

C:\Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\

From another device or the same old iphone...

Settings>icloud>Storage&Backup>Manage Storage, choose the device from the list, and on resulting screen, scroll to bottom and tap Delete Backup.