Although, i stand shoulder to shoulder with the above advice i.e. a clean install is always best practice. If needs must, you could choose the upgrade route - i.e. Windows 10 will literally integrate itself over the top of Windows 7 and it's existing software.
I've had to do this for a few systems (even with multiple drives) - and, indeed, when it was first released as a free upgrade - and so far not had any problems from users (although, systems have moved on in most cases). You would still need to back up important files (nuts not to) - but you should find yourself up and running in a few hours - all things being equal - and with all your apps/programs ready to use. Perhaps make an installation USB for 'worst case scenario'...
And this:
*That said, if you can do a clean install choose this option:- select create installation media (USB easiest), back up-back up-back up (
@bremen1874 advice is sound), format drive/delete partitions and do a fresh install (internet off, critical updates only during install - and all drivers directly from motherboard and GPU manufacturer.) *Recommend you only have the one drive attached during the install - Windows 10 likes to share :/