You can start a Windows 11 26H2 upgrade without a USB drive by mounting Microsoft’s ISO in the Windows installation you’re already using and running setup.exe. This upgrades the current operating system; it does not boot a blank PC or perform a clean install.
Before you start
- Check compatibility. Confirm that your PC meets Windows 11 requirements and check your device manufacturer’s driver information. Microsoft warns that installing on hardware below the minimum requirements can cause compatibility problems; unsupported devices may not be entitled to updates.
- Match the ISO to your PC. Choose the correct architecture—x64 or Arm64—and the same installation language as the Windows version currently installed.
- Make room and back up. You need enough storage for the ISO and an internet connection to download it. Microsoft does not state a universal free-space figure for this mounted-ISO method. Save your work and back up the PC before installing.
On Microsoft’s Windows 11 download page, the release is labeled “Windows 11 2026 Update | Version 26H2.” Confirm the release label and current known issues on the page before proceeding, since availability can change: Microsoft’s Windows 11 download page.
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Install Windows 11 26H2 by mounting the ISO
- On Microsoft’s Windows 11 download page, select the Windows 11 ISO download option. Choose the appropriate architecture and language, then download the file to a location with sufficient storage. Microsoft allows saving it to the computer or an external drive.
- In File Explorer, find the downloaded ISO. If you do not see a Mount option when you right-click it, open the file’s Properties, go to General, select Change, and set Windows Explorer as the app for opening ISO files.
- Right-click the ISO and select Mount. Windows will create a virtual disc.
- Open the virtual disc and double-click
setup.exeto start Windows Setup. - Follow the prompts in Setup to upgrade the current Windows installation. Available prompts and options can vary by device; check that your specifications and manufacturer drivers are compatible.
What this method does—and does not do
Mounting the ISO and running setup.exe from within Windows is an upgrade path for the operating system currently running on the PC. It is not a clean install on a blank disk, and it is not a way to start a PC that cannot boot into Windows. Microsoft describes bootable USB or DVD installation media for a clean install or reinstall.
When another installation route makes sense
| Goal or device | Suitable route | Important distinction |
|---|---|---|
| Upgrade the Windows PC you are using, without bootable media | Mount the ISO in Windows and run setup.exe |
Upgrades the current operating system. |
| Install or reinstall Windows on a blank or different PC | Create bootable USB or DVD media, as Microsoft describes | Running setup.exe inside the current Windows session is not a clean-install boot method. |
| Use Microsoft’s Installation Assistant on the current PC | Installation Assistant | Microsoft says it supports x64 processors only, requires a Windows 10/11 license, Windows 10 version 2004 or later, and 9 GB of free disk space. These conditions apply to the Assistant, not the mounted-ISO method. |
| Upgrade a PC with an Arm-based processor | Use the official Arm64 ISO path or follow Microsoft’s update availability guidance | The Installation Assistant is x64-only. |
Optionally verify the ISO download
If you want to check that the downloaded ISO matches Microsoft’s published file, use PowerShell’s Get-FileHash to calculate its SHA-256 hash, then compare the result with the corresponding value on Microsoft’s download page. The page does not specify a universal free-space requirement for installing by mounting the ISO; do not apply the Assistant’s 9 GB figure to this method.
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