How do I upgrade to Windows 10 if I have less than 8 GB free?

An Microsoft Employee explained it his show on channel 9. You need 2 empty USB flash drives. Put the Win10 files from the ISO on 1 drive. The 2nd one must be empty and during running setup.exe from the 1st USB drive, it shows you that you need more space, select here the 2nd empty drive as temporary drive (Choose another drive or attached external drive).