Can someone cure an ulcer just by changing your diet and taking antacids?

No not all ulcers: Most ulcers in stomach are caused by infection with h pylori a bacteriaand is treated with antibiotics and acid suppression, but few ulcers can be caused by nonsteroidal meds like aspirin, ibuprofen, narosyn etc and these may heal with diet and acid suppression with antacids and meds like Omeprazole and others.And rarely ulcers are due to cancer.The diagnosis is usually made by endoscopy.