I just did it today - it was super simple, here’s how.
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You need to remove the feet from the computer, remove 4 phillips screws, which reveal a further 4 phillips screws you must remove. then detach the Motherboard by removing yet another 4 phillps screws. This gives you access to the HDMI port (which they hide for some reason hence the need to opend the device).
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Connect the HDMI to a display, connect a USB keyboard and mouse and flash StartOS onto a USB using this guide Start9 | Flashing (x86_64).
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Plug in the USB-C power cable and immediately mash the delete key. This will open the BIOS. Use the arrow keys to navigate to the boot menu and ensure that the USB drive is Number 1 in the boot order. Press f4 on your keyboard so save and exit (or use arrow keys to navigate to save and exit).
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This will result in StartOS being installed from the USB onto the SSD in the Umbrel home, just follow the onscreen instructions. Eventually it will tell you to remove the USB and restart. You can now shutdown the device using the power button.
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Unplug everything and put the Computer back in the Case by reversing the initial steps. Now just plug in the device to power and connect to your router via ethernet and head to start.local and you’re done! [This shouldn’t take you more than 15-20 mins imo]