Do you already have Windows 7 Pro or Windows 8 Pro? To check: click Start and type winver [press enter] 

OK, what you need is to record your existing license key (written on the Windows sticker) then head on over to download Windows 10:

Download Windows 10 and choose "Create Windows 10 installation media"

...follow the steps to create a bootable USB (you'll need an 8GB USB stick)

Ensure that your files & emails - and any other data - is synced in the cloud (we recommend Office 365)

Insert USB stick into the PC and follow instructions to install Windows 10 (we advise a clean install - not an upgrade)

When asked for a license key enter your old key, and it will get converted to a new digital license!