(Redeeming serial hot #)
As far as I know they need to go to register.guildwars2.com. If they have an existing account they want to upgrade then select “Already have an account? Sign In” and click the sign in link. Then there is a link to add a code. IMPORTANT, they need to make sure they click the sign in link and not register via the form on the front register page as that will create a new account.
Disclaimer: I upgraded through the gem store so didn’t have to do this but this is what I read in another thread. However I’m pretty sure the instructions were sent via email with second key you purchased.
Edit: I found the link where someone posted the instructions that are sent via email.
https://forum-en.gw2archive.eu/forum/game/gw2/Question-about-code-sent-to-Email/first#post5165081
(edited by JustTrogdor.7892)