Remove back panel by:

inserting (flathead) screwdriver in the top and pressing this against the snap-fit;

pressing screwdriver on left side against the snap-fit simultaneously;

rotating back panel whilst pressing against the snap-fits to loosen the panel until it is no longer connected on that side;

pressing screwdriver on right side against the snap-fit and removing back panel altogether.

Expect difficulty with pulling, more hands may be required.