Method 1

With the following command

xrandr --output HDMI1 --rate 75

Where the string after --output is the name of the monitor and the number after --rate is the new refresh rate

Or, if you want to make sure the resolution is correct too in the same command

xrandr --output HDMI1 --mode 1280x1024 --rate 75

See man xrandr and the following link to learn more

https://wiki.ubuntu.com/X/Config/Resolution

Method 2

You should have a file monitors.xml (~/.config/monitors.xml)

Edit the file with your favorite text editor

vim ~/.config/monitors.xml

You will see monitors available, and you should recognize them by name.

Here's some example information you will see about a monitor

<vendor>AUO</vendor>
      <product>0x38ed</product>
      <serial>0x00000000</serial>
      <width>1920</width>
      <height>1080</height>
      <rate>60</rate>
      <x>0</x>
      <y>0</y>
      <rotation>normal</rotation>
      <reflect_x>no</reflect_x>
      <reflect_y>no</reflect_y>
      <primary>yes</primary>

There's the line

<rate>60</rate>

Replace the current refresh rate with a new one. For example

<rate>75</rate>

Save the file and reboot, and the new refresh rate will take place.

NOTE: Make sure that monitor's resolution supports the refresh rate you are going to change