Navigate to the gear icon in your navigation. You will arrive at a page for your Routing Guide Settings and Auction Settings.

Here you can set your default auction settings, click the gear icon in the auction settings box, and you can edit whichever settings you would like to, then click "Save".

These settings will be your default, but you can still adjust them for individual shipments when you're tendering them to auction, your default settings will remain the same.