SwitchResX is a utility for Mac that allows you to change the resolution of your monitor on the fly. This can be handy if you're trying to use a different resolution than what your Mac supports or if you want to change the refresh rate of your monitor. Here's how to use SwitchResX:
First, download and install SwitchResX from their website. Once it's installed, open the application and click on the Monitor tab. Here, you'll see a list of all the monitors connected to your Mac. Select the one you want to change the resolution of and then click on the Custom Resolutions tab.
Here, you can add custom resolutions for your monitor. To do this, click on the + button and enter in the width, height, refresh rate, and timing mode for your new resolution. Once you've added your new resolution, select it from the list and then click on the Apply button. Your monitor will now use this new resolution!