Using the Web Interface
To edit your credentials, go to the "Configuration" section of your device (https://X.X.X.X/configuration, where X.X.X.X is your instance IP address).
Then using the form, you can change username, password or even disable authentication if needed.
If authentication is disabled, please make sure that the virtual device is accessible only from authorized IP addresses for security reasons.
Using the command line
Connect to the instance using SSH or PuTTy (see Accessing a virtual device from SSH)
Change the username using:
setprop persist.webrtcd.username \$(echo -n "new_username" | sha1sum | cut -d \" \" -f1)
Change the password using:
setprop persist.webrtcd.password \$(echo -n "new_password" | sha1sum | cut -d \" \" -f1)
You can now use these credentials to connect to the web UI.
Disable authentication using:
setprop persist.webrtcd.authent off
For security reasons, make sure that the virtual device is only accessible from authorized IP addresses.
You can also refer to this tutorial to change or disable the credentials.