Change system defined media vault and media set in backup exec 2012

Hi!

I'm using backup exec 2012 (server win 2008R2). When I begun to use RDX data cartridges they were defined by the system to a media vault and a media set with infinite overwrite protection! This is not good because I plan to change the rdx datacartridge daily and the overwrite protection should not be more than 4-5 days. How do I change the system defined media vault and set to an other media set?

I hope you can help me :)