If you installed a standalone installation of Lightroom you should be able to access the registration through the help menu. If you installed from the creative cloud there is no way to license it with your serial number. It will have to be uninstalled, and then you can reinstall from the appropriate website. What operating system do you use?
Thanks for you advise. I have done that but I will have to say Adobe support has taken a turn for the worse lately
You have done what?