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.
Log in to the instance with SSH or PuTTY. Please refer to Accessing a virtual device from SSH
Change the property using
setprop persist.webrtcd.authent off
For security reasons, make sure that the virtual device is only accessible from authorized IP addresses.
Close your instance shell using
Refresh the web page running the virtual device.
You can also refer to this tutorial to change or disable the credentials.