pg_hba.confto allow you to connect without a password. Then use ALTER USER to reset the password. Then change pg_hba.conf back again.
For the documentation on pg_hba.conf, see here:
I'm not sure if this will help you, but here are the steps to reset a PostGreSQL password.
Have not tested myself !
Note that the upgrade installer , according to the documentation, is NOT supposed to prompt for password if you never entered one in the first place so something strange may be happening.
A look at the logs on the server may help too... they are under c:\programdata\qlik\sense\log (several directories under here each with logs). ProgramData is by default hidden by windows.