How to change how many cores my computer runs?

I assume you don't mean actually disabling cores, as I'm not sure as to why anyone would want that.

The number of cores you have are bound to the CPU, or processor, that is in your system. In order to have a different number of cores or threads in your system, you would have to invest in to a new processor for your laptop or computer.

Please don't judge a processor's performance on its cores, though. More =/= better. You'd want to look at other things, such as processor speed, CPU architecture, and perhaps most importantly, the socket, as if you don't get the same socket CPU as your current one, then it simply wouldn't fit, meaning you'd have to refund your purchase for a new one, or buy a new motherboard (something that generally isn't possible on a laptop, mind you).