Reducing Media Set Size

I am using Backup Exec 2010 R3.  I have a media set that was set to hold 8 weeks of retetention.  I now need space cleared on the drive its on (its not full yet, but i need space).  I have lowered the retention to 4 weeks.  The previous 4 weeks is not marked as over writable, but i need to delete it to free up space.  I beleive Backup Exec is using deduplication.  How do I safely remove the data 4-8 weeks to free up space?