If you previously bound an email address to your admin account during the initial setup, you can reset the password instantly using the mobile app. FAQ/Password Reset Instructions - DahuaWiki
The device was previously registered to an email address in the device’s settings. dahua password reset by serial number
Dahua provides a free utility called (often renamed to "DSS Express Search" or "SmartPSS" in older versions). This is the primary tool for a password reset by serial number. If you previously bound an email address to
shown on the device's On-Screen Display (OSD) to the appropriate regional support email (e.g., Support.USA@Dahuatech.com for North America or support@dahuasecurity.com for global support). Apply the Security Code dahua password reset by serial number