It's fairly straight forward to get into the t620.
Push down on the clip at the bottom of the back panel (by the power connector). This frees one end of the back panel and it can then be lifted off.
You can then see a green slide switch by the audio sockets. Slide this switch towards the power socket.
Sliding the switch removes the lock on the outer cover and you should now be able to slide it back about 5mm and then lift it clear.
Any comments? email me. Added April 2017