You can reset the password to the default number 12345678 by updating the firmware.
Note: As to A139, you can just reset the camera by following steps:
1. Pause recording, press the yellow button and WIFI button at the same time for several seconds until all the lights flash.
2. After all the lights flash, please do not press any button, the camera will restart recording automatically and WIFI SSID and password will all be reset.