No, Lightroom 6 is an upgrade that includes several new features. It is not a free update.
How do you purchase the Upgrade? I don't see the website for that just the full version.
Go to the following website: Products
Scroll to near the bottom of the page.
On the Lightroom line click on the Buy button.
Choose that you want an upgrade.
Indicate what version you are upgrading from.
Price will change to $79.
Complete the purchase.
Download and install.