On the side bar you will see "Deal Stages" click that link. You should now see "Sort Deal Stages" if you do look up a bit. Click on "Type a deal stage title". Type anything you want.
Click on the deal stage & you should have a popup box on the bottom come up that has a title of "Deal Stage Actions". Click edit & type in the box the new name.
You should see "Type a Deal Stage title". Press that box & start typing any name you want for your deal stage. When you are done typing click to the right a button that says "Add".
Press one of your deal stages. Now you should have a popup on iPhones it's on the bottom & on iPads it's in the middle. When that box pops up it should say "Deal Stage Action". Press the edit button on it. Now type a new name for the Deal Stage & click "OK".