How to get and install licensed windows 7?

How to get and install licensed windows 7?

HI! I had my laptop reformatted; I have a licensed windows vista before; I didnt realize that the tech has installed a fake windows 7..so i want to change into a licensed one, how do i go about it? can i just buy online then install it? can i install it alone or do i need tech assistance? thanks a lot in advance