To change the name, default servings or serving size of ingredients in your recipe you have to do so while creating it. Recipes cannot be modified after they’re created.
Whilst creating your recipe in the New recipe page, click on an ingredient in the list. You can now click on Adjust serving size and enter your number.
Then click on the back arrow to return to the New recipe page.
If you leave the New recipe page with clicking on Create recipe you’ll lose your recipe (but your selection of blue tick boxes in the Journal page will remain).