Within the item there is an option to set "custom bid increments". If there is not a bid increment entered, it will use the default bid increment structure that can be seen from the pop up link listed there.
1) Click "Auctions" on the far left.
2) Click on "Manage Auctions".
3) Click on the Name of your auction.
4) Click on "Items".
5) Click "Add Items".
5) If you choose not to use the default bid increments, click "Custom" and enter the new amount you want the bids to go up by.