When a baby has the stomach flu there's not much you can do to make the stomach flu go away any sooner but what you can do when your baby has a stomach flu is to make sure he or she gets plenty of fluids and is allowed to rest a lot.
If your baby has diarrhea or has vomited wait for 15 to 20 minutes after the vomiting or diarrhea before trying to give your baby the bottle or before trying to breastfeed the baby to allow the babies stomach to settle down.
Other than that there's not much you can do but just let the stomach flu run it's course until it's gone.