That should happen automatically when you plug in the mouse. If it doesn't, then you may have to connect it while the laptop is turned off, then turn it on. The mouse they should be the default cursor rather than the touchpad. At least until you unplug it.
You can also do the following, if for some reason the above doesn't work...
1. Go into "Control Panel".
2. Click "Hardware and Sound".
3. Now click "Mouse and Touchpad".
4. Now go to "Additional mouse options".
5. The Mouse Properties window will open. In here click on your device.
6. Make sure your device is set to disabled. If it isn't then click "Disable Device".
7. Exit both that screen and "Control Panel".