Urgent need help with Diarrhea and Diaper Rash

My 5 month old daughter having diarrhea since 15 feb 2021. Bring her to PD for monthly check up at NUHKids with Dr krish, he say most probably starting she has stomach flu but never seen doctor for it and it become lactose intolerance. So he recimmend Isomil soy infant milk to see and stop her diarrhea. Because of her every 1 hour diarrhea, her diaper rash bad to worse. How should I clean and get rid diaper rash and stop her diarrhea? I dont know what to do mummies!