The default is set the first time you run Contacts. Once you’ve set it, you can’t change it from the application. To change it, you must clear the settings, and run the application again.
The first time you create a new contact, Contacts will ask which account you wish to use to save it.
To clear the settings, go into the Settings -> Applications -> All.
Scroll down to “Contacts”.
Press “Clear Data”. It may take a minute to activate the button.
Press “Force Stop” to kill the executing application.
Go to the home screen, and find the Contacts app, and run it.
Press “+” to create a new contact.
You can now choose the account where it will be saved.