To preface this, I have and use an NVidia GPU, so I won't have any screenshots for AMD settings, but I will tell you how to do this if you have an AMD GPU.

NVidia
Open up NVidia control panel


Make your way over to the Display tab, and then change resolution


Now you'll want to choose the resolution you want to use, and click on customize, that will open up this;


Down the bottom of this window, click on "Create Custom Resolution" and that's going to take you here;

As you can see, I've set my refresh rate to 75Hz. I know my monitor can run at 75Hz stable, hence why that's what I've put in. I recommend bumping up the refresh rate incrementally, in 2Hz or 3Hz jumps. Now you'll test the resolution and NVidia control panel will ask you if you'd like to save the custom resolution if successful. If your monitor can't hold a stable overclock, your monitor will go black when you test the resolution. Don't worry about this, just wait 15-20 seconds or hit escape and try a lower overclock.

Once you've found a stable overclock and saved it, you can go back to the resolution tab in NVidia control panel and now you'll see a "Custom" tab under resolutions;

Apply your custom resolution and you're good to go.

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AMD
Open up Radeon settings(Crimson).

Go to the preferences tab, this should be on the bottom row in Radeon settings.

In preferences, open up Radeon additional settings. This should open up a new window that looks the exact same as the old Catalyst Control Center.

Now that you're in additional settings, you'll want to open up the "My Digital Flat-Panels" tab and go to "Custom Resolutions" down the bottom of this tab.

Now, in custom resolutions, you'll want to create a new custom resolution so just click "New". Now you'll be able to edit the horizontal and vertical resolution, as well as the refresh rate. Don't change anything else, just up the refresh rate and click "Verify"(instead of "Test" for NVidia GPUs). If your monitor can't hold a stable overclock, your monitor will go black when you verify the resolution. Don't worry about this, just wait 15-20 seconds or hit escape and try a lower overclock.

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If you want to check that your monitor is running at it's overclocked refresh rate, close all tasks and go here;
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