You buy it from Adobe at their online store. You should find a choice near the end of the checkout that asks if it is an upgrade. If you have the serial number from 5 and pick the slightly reduced upgrade price, then you will need both the old serial number and the new one you get when you pay for version 6.
You may be able to find the new version at sale prices that are about the same as the upgrade price. Many retailers sell it in a box with a disk. Many online retailers sell it too.
About 6 million people have chosen not to buy it but to rent it at $10 a month. You get both Lightroom and Photoshop. Find Adobe.com and look for the "CC" or Creative Cloud "Photographers Package".