You will need two CR2032 batteries, which is one of the most common type of coin cell and available anywhere.
1) Pry the housing open, which can be a little stiff (we erred on the “durable” side of the assembly fit!).
2) Remove and replace both batteries with the + side facing down in the pockets of the rear housing.
3) Snap the top back on, making sure all snaps are engaged so the gap is even all the way around.