To change the refresh rate (Hz) of the screen, you just need to right click on the desktop and click "Nvidia Control Panel" and under "Display" you select "Change Resolution" (although I think that's the default window it opens to).
Then from there you click "Customize..." and then "Create Custom Resolution" you will be then taken to a custom resolution window where you can write the "Horizontal Pixels" and "Vertical Lines" which is basically Width and Height, by default it's 1920 and 1080 and under that is where you change the Refresh Rate (Hz), I think you can safely write 75Hz there because it was proven to work on most of the G751J's and some models even comes straight out of the box at that refresh rate. You don't need to change other things unless you know what you're doing.
After adding that custom resolution, you can now switch to that at the Change resolution panel under "Custom".
You can go overclock further if you are brave enough, in 1 or 5 increments until your screen goes haywire or dead black, I would say the Green Zone is around 60-75Hz, orange zone is 75Hz-95Hz and anything above 95 or 100 is the red zone and could be dangerous. (You could break your monitor in any overclock, so do at your own risk, I'm not sure what are the odds but I haven't really heard anyone breaking their monitors from overclocking)
and as for MSI Afterburner, it's a free program you can get from the official MSI website, this is one of the most reliable GPU overclocking software that I even used for my desktop that have a non-MSI 290x. So don't let the MSI branding confuse you cause it generally works for every GPU (I heard even Intel ones, yes the video card included in CPUs)
