Okay, first of all. You can overclock. There's not much you can do or change, but hey, still there's something that you can do. The G752 series are pretty good at keeping temps low, so you can take your chances.

First, the CPU is the thing that can't be overclocked. Since, Intel has made sure that regular Skylake chips shouldn't be overclocked until and unless you have the 'K' chips with an unlocked multiplier. So people can shell out more money for the K chips and have to deal with less RMA's. So for mobile, you should have the 6820HK chip to overclock it to the fullest.

For GPU, you have two tools at your disposal. ASUS' own GPU Tweak or Gaming Center or 'whatever they call it every month'. It should allow you to overclock on a certain limit and there's not a lot you can mess around with. Just a couple of sliders to move. Still, that should be enough for beginners if you know what values you can mess with.

On the other hand, there's a tool called MSI Afterburner (which I currently use) which has the same set of sliders but it gives you the freedom to play with voltages and memory clocks as much as you want. (I have no idea on messing with voltages though). This is the same tool that is used for overclocking desktop chips as well.