It is possible to remove the back cover with an iOpener as seen in this guide but it's a lengthy and difficult procedure. We recommend a heat gun or something similar. The adhesive in the Nexus 6P is extremely sticky and especially the glass and plastic cover on the backside break or bend most of the time whilst doing this repair.

Use a SIM card eject tool, to pop out and remove the SIM card tray.

Use an iOpener to loosen the adhesive underneath the small plastic cover at the bottom of the phone.