How to delete a Backup from a backup set in SQL Server 2008 R2

I would change your backup strategy to write to individual files for each database backup. You could then have a maintenance plan step that examines your backup location and removes *.bak files older the X days.

There's no way (that I've ever heard of) to delete a single backup from a backup file. This is why people write each backup to a separate file so you can just delete the old files.