You pull the Windows 7 hard drive out, you buy a new hard drive or solid state drive. You install that and you install 10 on that. That`s how you do it.
You do that when you can afford it. Or you upgrade to Windows 10 when it comes out through windows update, but I don`t even recommend that.
Just get another hard drive for the desktop and put windows 10 there, don`t risk the laptop.
Or upgrade 8 on the desktop.