Re: cant install windows 10 on lumia 520 due to low memory space
You can move your data to your SD Card. Use the Storage Sense app to move your apps, and as for photos/music/videos/documents, you can use any file manager app to move them.
If you do not have an SD Card, you'll have to uninstall your apps and make a backup of your media on a PC. Once you install Windows 10, you can restore the data and the apps.