If upon hitting the power button or selecting “shut down” on the start menu, your system reboots, then these steps will likely solve your problem:
Step 1: Press start, and go to settings.
Step 2: Select System, on the top left.
Step 3: Select “Power & sleep, on the left navigation menu
Step 4: Select “Additional power settings”.
Step 5: On the left side, select “choose what the power buttons do”.
Step 6: Click “Change settings that are currently unavailable. You may be prompted to enter your password. If you are a standard user, you may need someone with an administrator account to complete this for you.
Step 7: Uncheck “Turn off fast startup(recommended)”
Step 8: power off your system to verify that shuts down properly instead of rebooting.