This is targeted to those who have bought the original game via Microsoft Store. The Definitive edition free for the next 2 days which is a great deal for a great game.
Should be able to redeem through the Xbox Windows or Android app or via the Microsoft store.
If you purchased the game via another countries Microsoft store, change the link to that country if it doesn't work via the Australian store.
NEW IN THE DEFINITIVE EDITION
• Packed with new and additional content: New areas, new secrets, new abilities, more story sequences, multiple difficulty modes, full backtracking support and much more!
• Discover Naru’s past in two brand new environments.
• Master two powerful new abilities – Dash and Light Burst.
• Find new secret areas and explore Nibel faster by teleporting between Spirit Wells.
EDIT: Sorry not 100% sure on the free upgrade on the PC, might not be free via PC so I've removed it from the description. Only thing I can see as posted in the comments too, is that it can be upgraded for $5 with a coupon.
Link to Steam
Link to How to Find Steam Coupon