Instructions
Check your map for a neutral harbor.
Click the harbor. It will give you the option to end your patrol at any harbor, but you will only be able to resupply if there is a resupply sub at your chosen harbor. You will know it is there if you are given the additional option of resupply.
Click "Resupply." It will end your patrol, but you will but resupplied. As time passes in the simulation and Germany begins to lose the war, resupply ships and subs will become more rare.