It won't recognize your LR5 install if that was also an upgrade version of LR. Then you have to give LR6 install the serial number of the full version you first bought. If you don't have that serial number, then you'll have to call Adobe. I don't know that they'll help you though. A lot of people get burned when they lose their Adobe software serial numbers--that can be just like losing the program altogether as far as upgrades or reinstalls are concerned. (This is an advantage of buying direct from Abobe now; they keep your serial numbers in your account online.) If you can't get a serial number from them, you would have the option of returning the upgrade edition you bought from Adobe to Adobe within 30 days for a refund and just using LR5, or paying for either a full version of LR6 or subscribing to CC--not ideal, I realize, but at least you're not out the $80 you just spent on the upgrade edition as well.
@lauries you are not doing an update to LR 5, LR 6 is a new version. You need to download the perpetual license version (do not use the CC application manager), run the install package and when prompted enter the 24 digit numeric license key you got for the upgrade, if LR 5 is installed on the same computer the previous license key should be automatically be recognized if not you will be prompted to enter it so make sure you have it available for this process.