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.
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 2560×1080):
xrandr --output HDMI-A-0 --mode 2560x1080 --rate 74.99
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”.