Move the throttle to the position marked "Stop" and turn the snow blower off. Remove the spark plug wire so that you can't inadvertently start the machine and cause injury.
Wait for all of the moving parts to stop and grease the auger shaft fitting with a grease gun -- you should be doing this every 10 hours anyway. Be especially sure to do it when you change the bolt.
Remove any debris and line up the hole in the auger with the hole in the auger shaft. Install the new shear bolt. Only use a shear bolt -- if you use any regular bolts, you'll defeat the safety mechanism, and any debris collected by the auger while snow clearing might destroy the auger or machine.