Was the bundled version of LR5 pre-installed? Did you register the original product with Adobe? If so log-in to your Adobe account and choose Manage Account. You may find the serial number in your product history. Otherwise you will need to contact Adobe directly for help to upgrade to Lightroom 6. Click on the blue button: Still need help? Contact us – then click in the area marked “chat 24/7”
It’s usually possible to start a live chat, if an Adobe agent is free, and often to get the problem fixed right away.
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If you want Lightroom 6 you will need to pay for an upgrade from Lightroom 5 or purchase a full license o Creative Cloud Subscription.