You need to login to your Adobe ID with the computer you wish to deactivate, your user name would be the e-mail address you used to create your a/c. If you no longer have access to the computer you will have to seek support directly from Adobe support at Adobe dot com and look for support or star a chat. Make sure you have all your info available including your serial number of Lr 6, if that was an update, you will also need the serial number from the earlier version.
If you are able to log in to your a/c you should be able to see the info on the Products you purchased and registered. I have deleted my serial numbers for earlier versions of Lr versions 1 & 4. and an earlier version of PS.