Use a plastic opening tool to free the clips on the top and each side of the back cover.

The bottom is secured with some adhesive foam. You should be able to finally free the back cover using an opening pick. The "snaps" at the bottom edge are more like hinges, so release all others then swing the back cover off outward from the top, and these fasteners will release without breaking.

It's best to avoid prying around the SIM card slot and volume controls, as the plastic in these areas is very thin and likely to break from prying.

When you replace the cover, there is a snap about even with the flash window that must also be attached (one of three that secure the back cover to the midframe). Qi (wireless) charging may not work if this isn't secure. Apply gentle but firm pressure over the "S" in "Nexus" and slide your thumb up towards the top, and the snap will engage.