How to reset or get back the root password of Huawei 5683T?

A1， you can try to Load the empty database matched the device again through DoS. After restarting the device, log in to the system by using the default root password admin (OLT) or mduadmin (MDU). This method will lead to loss of data configurations and the user needs to configure the data again.

A2，I have another way:
Modify the configuration file using the [url=http://www.thunder-link.com/support/U2000V100R006C02SPC301-Installation-Introduction_Part%201.html]U2000[/url].
On the U2000, choose Administration > NE Security Management > LCT User Management from the main menu.
On the LCT User Management tab page, select NE User and then Device Type. Select the desired device name from the Device Name column and user name from the Name column. In the displayed dialog box, choose Set Password to set the new password.