There is a drain plug inside the tunnel, No. 16 towards the right side of the attached image. It is always good to check the condition of the chain and gears and this is the perfect time, so remove the cover and give things a look over before refilling. It can be easier to set the chain tension with the cover off, certainly if haven't done it before, so you can see what is happening when you turn the adjuster. You can fill it through the fill hole near the top of the cover, or through the dipstick hole further down.