The Upgrade Advisor app will tell you if your Windows Phone 8.1 device is eligible for the upgrade to Windows 10 Mobile. If you're eligible, it can also help free up space on your device so you're ready for the upgrade.
Search for the Upgrade Advisor app, and then install it on your device.
Open the app on your device. It checks your device and lets you know if it's ready for the upgrade, if an update is required before you can get the upgrade, or if your device can't be upgraded.
If your device is eligible for the upgrade, you may need to free up some space to get it. The app will recommend files, such as videos or photos, that you can temporarily move to OneDrive or an SD card, if you have one installed. Accept the recommendations, or change which files you want to move. You can also delete some files, if you want.
If you move files to OneDrive, you need to use the app to restore files back to your device after Windows 10 Mobile is installed.