This usually involves removing one or two small screws, taking the heatsink off, and then your M.2 slot will be ready to receive your SSD.
Once you've located the slot you want to use, you may also have to remove a heatsink / cover to actually get to the M.2 slot and install your drive. This usually involves removing one or two small screws, taking the heatsink off, and then your M.2 slot will be ready to receive your SSD.