Is it possible, with Youtrack 6.x, to:
So far, I'm only able to change passwords, one user at a time.
This question is old, but in case someone else comes across this using YouTrack 2024
To set password a password policy go to Settings > Access Management > Auth Modules, then choose "Hub". From there, set your desired policy
https://www.jetbrains.com/help/youtrack/server/set-a-password-policy.html
To force all users to change their password, go to Settings > Access Management > Auth Modules, then choose "Hub". From there click on "Require password change"
https://www.jetbrains.com/help/youtrack/server/request-password-reset.html