After purchasing Lightroom 6 I found that I cannot locate my serial# for Lightroom 5 in order to activate it under the upgrade option. It is not listed under my account as a registered program, although I did register it at one time. In fact I updated LR 5 several times from 5.0 thru 5.7. I read that you can find the serial # under Windows at c:/users/program date... etc. I found the .lrreg file that supposedly contains the serial # but it cannot be opened. Does anyone know how to open this file. It seems to be specific to Lightroom.
Use any text editor
By the way, if I may be so bold, you really ought to have your serial numbers (for all software, not just Lightroom) written down and stored somewhere, or in a text file that can be stored in the cloud so you can never lose them.