How to open the extended sheet battery?

Hello everyone,

unfortunately, my external battery has only ~80% of its capacity. Other people have the same problem, that a soldering point between the cells is broken.

In the Sony support forum, one user was able to open the battery, but I'm scared to damage a cell. (see https://community.sony.com/t5/VAIO-Hardw...)

Can someone please explain me (maybe with pictures, where to pry open), how to open the sheet battery?