What is the earliest I can safely stop my daughter taking Flucloxacillin antibiotic?

My daughter has ben incorrectly prescribed too high a dosage of an antibiotic and I cannot reach the Dr tonight. She is taking 250mg (5ml) every 6hrs and for her weight it should be closer to 200mg. She is having stomach upset. Is it safe to stop anytime, or reduce the dosage or is it best to finish the course?