Your question is very vague. How to design this is largely going to depend on what you are attempting to change these user passwords, but the general instructions would be something like 'display form asking for new password, then submit that new password to whatever end application you are using.
I am not sure you are going to get many others with experience with this, I am not aware of any clients that use Process Automation in this way as this is really not its intended role. Process Automations role would more be to call the application in question and facilitate and help automate the password change action not request and make the change in the 3rd party app.