FAUTLY BATTERY TOOTHBRUSH UNDER WARRANTY

A FAULTY ORAL B PRECISION TOOTHBRUSH! Bought under 18-Months ago, replaced batteries 2x AND NOT working. How do I replace?.....PLEASE HELP!!!!!!!! I am desperate for a response from Oral B on what I need to do because I have never used any toothbrush but Oral B and had no such issues before!