I have now installed Win11 on a Dell laptop (2016 vintage/model; 6th gen i7) which is not officially supported. IMPORTANTLY none of this is destructive; all the files and folders and apps are still in place and seemingly working. Device Manager shows nothing broken or unknown or...etc. It already had the most recent Win10 (21H1) on it.
I'll write this up in more detail if/when I have more time, but in summary I had to do all of these:
First up, obtain Windows 11 installer and put the files onto a removable drive (SD card, USB stick, etc.) Here:
Download Windows 11 (section: Create Windows 11 Installation Media). Put it to one side. Now to the machine itself:
1: Back it up. Take a full system image and ensure you have bootable external media (mine is a CD in fact) so it can be fully restored if necessary. I used Macrium Reflect Free for this.
2: Find out how to access the BIOS on reboot. (On my Dell it is repeated pressing of F2 on restart - but they are all different). Reboot and enter BIOS; check the BIOS allows for UEFI and Secure Boot - but DO NOT change it yet. Exit and start Windows.
3: Add the above (post 12) entry to the registry (regedit).
4: Check the partition type on the main disk drive; if it is MBR (Master Boot Record), it needs to be changed to GPT (GUID Partition Table). (Create and Format Hard Disk partitions / Right Click on the main drive > Properties > Volumes tab > Partition Style).
5: Change it if necessary: Reboot it to a command line thus: Settings > Update&Security > Advanced Startup > Restart Now > Troubleshoot > Advanced Options > Command Prompt. Run the command
mbr2gpt /validate first to confirm and then
mbr2gpt /convert to convert. DO NOT attempt to do this from a command line prompt from within windows.
6: CARE: Exit and restart - but interrupt the restart to enter the BIOS. (It won't boot into Windows).
7: Change the BIOS settings to UEFI Boot and enable Secure Boot. Make careful notes of all the changes made (in case you need to revert). All BIOS differ so you will have to look for these settings.
8: Save & Exit BIOS and restart. The machine should now boot into Windows 10 as before.
9: Insert the Win11 installation media and run setup.exe. When you are presented with a warning about machine specs, "Accept" the risk and continue.
E&OE. Your mileage may vary. And note Microsoft's official warning:
Microsoft recommends against installing Windows 11 on a device that does not meet the
Windows 11 minimum system requirements. If you choose to install Windows 11 on a device that does not meet these requirements, and you
acknowledge and understand the risks,..........