This battery change is easy! Use a CR1620. The important bit is to identify the lug to depress allowing separation of the key assembly from the fob. The bradawl is pointing to this, depress it and push fairly hard then pull the two halves apart. Remove (prise off) the lid of the fob at the central hole under the lid - images 5 and 6 or prise off at the edge (image 7) and hey presto. Some good youtube videos of this. Apologies for sl blurred images - I don't have enough hands!!