Looks like the standard Shimano one. Squeeze the ends of the circlip together - you'll probably need circlip pliers to do it - remove the circlip and the washer behind it, remove the battery, and you should then be able to remove the plastic housing.
Looks like the standard Shimano one. Squeeze the ends of the circlip together - you'll probably need circlip pliers to do it - remove the circlip and the washer behind it, remove the battery, and you should then be able to remove the plastic housing.