On the side bar you will see "Deals" click that link. You should now see "Click a deal stage header" on the top bar. Click on a deal stage header. Start typing your deal name & then click enter.
Click on the deal that you want to delete. Over on the left sidebar you should see Deal Deal red button.
Confirm delete with clicking the
Yes, delete it! blue button.
You should now see a box that says "Add a deal to (This is your custom deal stage name)". Press on the box & start typing your deal name. When you are done press the "Add" button on the right.
Right now deleting is not supported on mobile devices.