Don't.

First off, it won't be a free upgrade; you'll have to buy it (probably $119ish) but here's the short answer:  You'd be installing a new OS on a system probably 5-10 years old.  Your money would be better spent on a new system.  And there's a very good chance that you might not be able to get device drivers for your older hardware - printers are a common problem.  When I moved from 7 to 8, my own printer's manufacturer did not make upgraded drivers for a working-just-fine printer.