Normally the process is to uninstall your serial-number LR 5.5, then quit and restart the Adobe Creative Cloud application, at which point it’ll show the LR 5 update in its CC apps list, and you just install that; however, there was a problem with the LR 5.5 installed that way in that it wouldn’t’ start, and the solution was to install the serial-number version, again, which should pick up the CC serial number and be ok.
It is possible that they’ve fixed whatever the problem was on the first day, but be ready to install things a couple times.