Paste this into the terminal to see what refresh rate options you have. The rate marked with "+" is what is recommended, "*" is your current refresh rate. 

<code>xrandr</code>

To change the rate to, say, 75 Hz write this (in case you use HDMI - this information will show with the previous command. It is also important to write the exact screen resolution. Here is 2560x1080): 

<code>xrandr --output HDMI-A-0 --mode 2560x1080 --rate 74.99</code>

However this is valid as long as you don't reboot or shutdown your system. 
To make it permanent, put the second command into the app "Startup Applications".