Android - How to remove Google Account that is set for Android Market in device?

You can't currently remove the primary Google account that you used to set up your Android device, the quickest way to switch to your new Google account would be to restore factory defaults and set up your device to use the new account.

The location of the factory reset depends on the Android version of the device. On ICS (4.0+) devices it's in Settings -> Backup & reset -> Factory data reset. On Honeycomb (3.x) devices it's in Settings -> Privacy -> Factory data reset. The same goes for Froyo + Gingerbread (2.0-2.3.x) if i remember correctly.

Note that this will clear your apps and their data, and depending on your device, MAY wipe your internal and/or external SD card, as TryTryAgain mentioned.