Here’s the video showing how to disassemble a modern Thinkpad to replace the battery. You might notice that it looks a fair bit like the video to replace the battery on a modern phone: https://youtu.be/lpMGVM8lPTU&t=1m10s

(Also interesting that it doesn't appear that Lenovo even sells the battery [though several 3rd party suppliers do]: https://www.lenovo.com/us/en/d/compatibilityfinder?accessory... )