There are only two methods to reset the Proface HMI server password and both require access to the local HMI. You cannot do it using the Remote Client software.
1) On the physical HMI, navigate to the Main unit settings >> Remote viewer settings >> Proface Remote HMI Settings. There will be a Connection Password box, where you can enter your new password and save it.
2) Using GP-Pro ex software and the backup of the project already in the HMI, go to the Display Unit option >> Remote Viewer. There will be a Connection Password box, where you can enter your new password and save it. You then need to transfer the modified project into the HMI. This requires your PC (with GP-Pro ex) to be connected to the physical HMI (via ethernet or usb).