There may be a time when you need to reclaim storage space that is currently held in retention for a specific backup set. This can especially be true for "large file" sets such as Exchange or Windows images.
The cleanest way to accomplish this is to set the retention period to "1" day for the backup set in question. This is done in the "Retention" section of the backup set settings (Launch client -> [Settings] at bottom -> [Choose backup set] -> [Retention]), or via the [Backup Sets] section of the management portal
Items in retention for the backup set will be DELETED approximately 24 hours after the next backup is performed.
If you absolutely must have the data deleted immediately please contact support.