Instructions
Launch WillowTree. The application opens in its own window.
Click the WillowTree logo icon located in the top-left corner of the window. This opens a drop-down menu.
Click "Open" in the drop-down menu. Locate the "Borderlands" save file you wish to modify. This is normally in the "Savedata" folder in your "Borderlands" installation directory. Select the file, and click "Open." The top of your WillowTree window will display the information about your character.
Click the "Ammo Pools" tab located at the top of the WillowTree window. This displays a screen with available weapons on the left and the properties you can modify on the right.
Select the weapon for which you wish to change the SDU level.
Assign the SDU upgrade level for the weapon by either clicking the up or down arrows beside the value, or clicking in the "SDU Upgrade Level" field and entering the number manually. Repeat this for each weapon you want to upgrade.
Click the WillowTree icon in the upper-left corner. Click "Save" to save your SDU settings.