Basically you just have to re create the account_new page (in the administration module). There you will see that the entity AccountPasswordData is used. Then the save microflow set the password to the account entity.
This is the page I was talking you about.
So in your Form create a new Button “Change Password” and call the follow Microflow below.
If you are using the Mendix Studio Pro try to find in the App Explorer the “ShowPasswordForm” Microflow:
and call it in your button.
this microflow will do the follow steps:
I hope it can help you.