Select the recipe from your Custom Recipes List in the Foods tab or by searching for your recipe's name. This will open the recipe in the food editor. Edit the text of the name, servings, add/remove ingredients and save your changes. When you save your changes you can apply the changes to past entries in your diary, or save a new version of the recipe going forward only.
If you use a recipe a lot, but make substitutions or alter serving sizes each time you make the recipe, we have a better option for you. Go to your Diary tab, add the custom recipe to your diary. Right-click the recipe in your diary to bring up the menu, and select Explode Recipe. This will break down your recipe into each of the ingredients (based on the serving size you entered). Then you can edit/delete foods to quickly modify the recipe each time.