With the Azure RM portal, resetting the Built-in Administrator account is not a difficult task. But this feature will only support for the Local admin account password reset.
To do this you need to go to Support + Troubleshooting > Reset password. This can be found under the Virtual machine.
Under mode, you can select "Reset Password" or "Reset Configuration Only". Then the selected blade will be displayed.
If you want to Reset the Remote Desktop service configuration, then you can select the "Reset Configuration Only".
To reset the password, The VM need to be up and running. If not this service will not be available.