Why in hell would you want to disable some cores? Doing so would kill the performance of your computer.
It often, but not always, can be done from the BIOS setting.
If you want to do that because it's overheating, you should find what prevent correct cooling, like to much dust, a faulty fan or using the laptop on a bed or cushion/pillow...
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix notebooks/laptop pcs.