How can I take the back off

Brute force is not the recommended option! The lower case is secured by hidden latches, that you preferably use a suction cup and opening pick to remove.

Lenovo has official guides on how to disassemble and replace most parts of your laptop, which you can find in the Hardware Maintenance Manual, see page 27 for how to remove the lower case (i.e. chassis/ the back).

Your laptop only has visible screws (seven screws of different lengths!), then the lower case is secured by hidden latches that you have to unsnap.

Your laptop has latches all around the sides which you can use a opening pick / guitar pick / credit card to insert in the crevice between the lower case and the chassis to pop open the latches.
Then you will have a latch in the center of the lower case, which you are to use a suction cup in order to undo. If you do not have a suction cup, I would really recommend getting one, they're really useful. You could use brute force to force open the center latch, but you run a moderate risk of damaging the latch in the process, and therefore I cannot recommend it.

For reattaching the lower case, you simply put the lower case back, then press along the sides and in the middle to secure the latches again.

For a visual demonstration you can follow this iFixit guide with clearer steps, the iFixit guide is for another model but judging from the Hardware Maintenance Manual the steps are pretty much identical.

Feel free to ask further questions, or if you want any clarifications.

Good luck,

Cheers