Change default routing on an Cisco SG300 switch

I want to build a high speed router VLAN with one device the router so there is no slow downs from other devices in the same VLAN. Some devices get chatty. My switch is in layer 3 mode.

I just noticed a cannot change the default route on my SG300-28 switch. When I select it I cannot use the edit button. It is grayed out. Is there a simple way to do this? Am I missing something?