Resetting passwords in PrivMX is possible only for “managed accounts” and can be done only by logged in PrivMX admins (in the Server Admin window).
The process of resetting passwords for “managed users” is possible, because admins keeps mnemonics (Recovery Keys) of such users in their encrypted PrivMX storage.
When admin clicks “reset password” for an user, admin’s computer dowlnoads and decrypts saved user’s Recovery Key, uses it to login as the user and change password to new temporary, random one. Then, this quick session with the user’s account can be closed which ends the process. The new password should be given (somehow) to the user. That’s all.