Here’s the procedure to follow if you want to upgrade your mobile device OS from Windows Phone 8.1 to Windows 10 Mobile.
Upgrade is not performed through system updates. Instead Upgrade Advisor App is required.
Not all Windows Phone 8.1 devices can be upgraded to W10. For an up-to-date listing check here.
Official documentation from Microsoft on how to upgrade is available at here.
Suggestion: perform all upgrade steps when device is connected to Wi-Fi network (lot of data downloaded during upgrade)
Step 1: install Upgrade Advisor App
On your device download and install the Upgrade Advisor App via Windows Store.
Step 2: check and perform upgrade
Once the app is installed execute it. When you open the app, it checks your phone and lets you know if it’s eligible to install the upgrade, if an update is required before you can install the upgrade, or if your phone can’t be upgraded.
If your device can be upgraded to W10 Mobile then run the upgrade. It will take up to more then 1 hour for the upgrade to complete (depending on connection speed, device performance, …). App will download W10 and then setup it.
Step 3: reconfigure apps (if required)
You’ll probably need to reconfigure some apps (i.e. Outlook)
Step 4: download Windows 10 updates
Once upgrade is complete you’ll probably need to download W10 updates (500+ MB).