On the FoodMarble app, open the Home page and click on your Breaths & Meals graph. This will allow you to bring up the Daily overview screen.
Once you have the Breaths & Meals overview open, you will be able to locate a pencil icon on the top right of your screen. Click this!
After clicking the pencil, you will have an option to delete any meal or breath test logged that day by clicking on the X next to the entry you wish to delete.