open your terminal and type this

sudo apt-get install compizconfig-settings-manager

then type compiz in your unity dash , it will list you compiz settings manager .Open compiz and at general options click at composite and there you have untick the detect refresh rate then you can manually set the value you want . .

and two know allowed refresh rates for the resolution you can use xrandr

the second row are allowed refresh rates with respect to the resolutions . so by depending on resolutions you have set , you can change the refresh rate by using the above methods .