I have a Moto G 2nd gen with 5.0.2, and after multiple accidents, partly due to fat fingers, the 6.0 update is now ready to be installed, but I do not need or want 6.0.
Welcome to Android Central! What screen are you on right now? If the installation hasn't started yet, can you tap the Back or Home button? You could always force a shutdown as well by pressing and holding Power for 10-15 seconds -- but make absolutely sure the update hasn't started installing.