The Windows 10 Mobile Upgrade Advisor app will tell you if your Windows 8.1 phone is eligible for upgrade to Windows 10 Mobile. It can also help free up space on your phone so you’re ready for the upgrade. When you open the app, it checks your phone and lets you know if it’s eligible to upgrade, if an update is required before you can upgrade, or if your phone can’t be upgraded.
If your phone 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 phone after Windows 10 Mobile is installed.
The app is still in beta though, and isnt available in all regions yet. However, we already know which Lumia phones will be the first to get Windows 10 updates. The update rollout has began already, and both the 640 and the 535 are already getting updated in two regions. Download Upgrade Advisor Beta