Choose "Enable Windows 10 upgrade" and follow the prompts.
The program (found on trusted community forums like GitHub or XDA Developers).
For historical reference, the process to use the app was simple and free:
Run the tool on your PC to push the Windows 10 Mobile update files directly to your phone, bypassing the need for the mobile app store.
Because the phone cannot install the file directly from the file explorer in version 8.1, you need a deployment tool.
Seeking the app today should be done with care: installing legacy system utilities from untrusted sources risks malware or incompatibility.
While the official "free download" of the app is gone, tech enthusiasts have found alternative (though unofficial and potentially difficult) ways to move from Windows Phone 8.1 to Windows 10 Mobile: How to update Windows Phone 8.1 to Windows Mobile 10
However, millions of users were stuck on Windows Phone 8.1. The official "Over-the-air" (OTA) update via settings often showed "Your phone is up to date" even though Windows 10 Mobile was available.
