Hi Damir,
On the page mentioned above you need to click on 'Buy Now' at the right hand side as shown in the screenshot below :
After that you will see this screen :
Click on 'Subscription' drop-down and select 'Upgrade', The other dropdown menus will be updated accordingly and you need to select 'I own' as 'Acrobat 9 Pro,
Platform and Language accordingky.
Then you can 'Add to cart' and checkout.
Regards,
Rave