How do you cure an upset stomach?

More specific please. "upset stomach" is not enough information to help you. Do you have nausea with/without vomiting, fullness, bloating, gassiness, pain (if so in what location(s)), bleeding, diarrhea/constipation/change in baseline bowel habits, tenderness, distension? Are you pregnant, have underlying disease, >60?
Nausea? Consider the traditional chinese medicine tonic- ginger. You can make fresh ginger tea by pouring boiling water over freshly sliced organic ginger. Steep in a covered pot for at least 5 minutes. Cool and sip slowly. Slightly flattened ginger ale or crystallized ginger also work. You can also use ginger in cooking. For severe abdominal pain please seek urgent medical evaluation. Take care.