Password could be changed by administrator. There are two ways to do it: web interface and console wizard.
Administrator can change password for any user by going to Settings > Users, then selecting the user's account and clicking on the action menu to choose the 'Reset Password':
You may use console wizard to reset own password. On your server launch next command:
alta reset-password
Wizard ask you next details:
If you have any issues with console wizard please check common errors:
Service is not started or installed on another server
Service is more privileged than wizard. Launch command with sudo to raise privileges:
sudo alta reset-password
User with defined name is not found