Not sure about v5, but v4 used to store some settings regarding to that in the registry. Maybe you'd like
to check following keys:
\HKEY_CURRENT_USER\SOFTWARE\Simplify3D\S3D-Software\MainOptions
"cloudRememberMe" (true/false)
"cloudToken" (some kind of hash?)
"cloudUsername" (nothing but my regular username in there)
"encCloudPassword" (your pw in encrypted form?)
Obviously v5 might handle all this very different, but maybe simply removing a key like cloudUsername
will re-create once you entered user/pw. Don't forget to backup the hive/keys you're gonna change
Cheers,
Daniel