To disable Fast Startup in Windows 10, follow these steps:

1. Open the Start menu and type "Power & sleep settings," then open it.
2. On the right side, click on "Additional power settings."
3. In the Power Options window, click on "Choose what the power buttons do" on the left panel.
4. Click on "Change settings that are currently unavailable" at the top.
5. Under "Shutdown settings," uncheck the box for "Turn on fast startup (recommended)" to disable it.
6. Click "Save changes" to apply.

This will turn off Fast Startup, which is a feature that helps to start your PC faster after shutdown by saving the system state. Disabling it means your PC will go through a full shutdown process. This can be useful for troubleshooting or if there are issues with updates or dual-boot systems.