I overcloked my imac late 2013 21,5" with the intel control panel, i don't recommend to do it and usually i don't even use the overclocked profile.
What i did:
1) Open the intel control panel (iris pro graphics in my case) or graphics properties (you can see it on windows bootcamp by right clicking on an empty space on the desktop)
2) Go to screen
3) Go to custom resolutions, the last option on the left at the bottom before "select profile"
4) Click on "YES" when the warning opens.
5) Choose add and put the dimensions of your monitor (the resolution)
6) In the update frequency, try from 60 to increase of 1/2 hertz, every time you need to click on "add" on the right bottom + button.
7) After adding your customs resolutions, reboot your devide
8) Now open again the intel control panel (graphics properties)
9) Go to screen again
10) In general settings choose the resolution and one of the update frequencies.
11) Check if the settings got effect here: https://www.testufo.com/
12) The hertz and FPS should be the frequency you choosed.
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DISCLAIMER: this is only for intel integrated graphics, for others the tutorials on the internet should be fine. I don't recommend to do overclock on a monitor, even worse if integrated with the PC because hard to replace.
Try at your own risk.