If you really want the free upgrade from Win7/Win8.1 to Win10 right away (after it launches later this year), you should first CLONE your laptop or desktop to a SSD (Solid State Drive) so you can switch the drives back just in case there are early issues/bugs with Windows 10 OR you find one of your key programs or add-ons is incompatible.
Running on SSD is VERY fast, never needs defragmenting, and is durable --you can't have a traditional disk drive head crash because SSDs have no moving parts (they're built from flash chips).
The cloning kit below comes with an external enclosure so you can re-use your existing laptop-size drive as a backup. Ask Professional Nerds if you need a larger enclosure or have any other questions.
$67 Thin 7mm 120GB SSD ($95 / 240GB SSD) w/9.5mm adapter
Read/Write speeds of 500/500 MB/s
Note: there are cheaper SSDs, but they're usually
SLOWER, especially WRITE speeds - avoid PNY!
$37 Hard Drive /SSD Upgrade Cloning Kit