My 8 month old had an underarm temp of 100.3 what should I do?

Watch for now. I would not do anything t this time. Watch for further symptoms. If the baby is doing ok with eating, sleeping and disposition wait to see how things develop the next 24-48hrs. It the temp is over 101f may consider a fever reducer. If over next 1-2 days she still has a temperature over 100.5 f or higher have the doctor look over things and consider a urinalysis.