To edit a recipe that is not your own, you must first duplicate it into your Cookbook.
On the recipe page, click 'More Options...':
Click 'Make a Copy' to duplicate this recipe into your Cookbook.
You will be directed to the edit page for the copied recipe. By default, '(Copy)' will be added to the title.
Change the title to whatever you like. Go ahead and make any edits to the recipe, and click 'Save Changes' when you are finished. The edited recipe will be saved in your Cookbook.