Get into BIOS and look for some relevant setting. It could be controlled/locked from a BIOS setting 
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There isnāt any.
Might be worth mentioning, that setting 60Hz works on a system where I donāt have NVIDIA drivers installed. Using nouveau wll allow me to choose any framerate I wish. So I think itās something to do with the NVIDIA drivers.
Now, how and if this helps me I cannot say.
Switch the display to Intel and then try switching to 60 Hz.
Look into your laptop user manual. It is a hardware related issue. Any hardware or firmware info, specifications could be helpful.
I agree that nvidia settings could have some configuration, but itās you who has to try them
The funny thing is that usually users try to raise than lower monitor frequencyā¦ 
From what I found some models support switching the refresh rate with the key combo Fn+R.
If this is not working it looks like you need third party software. There is something called CRU (Custom Resolution Utility) but itās just for Windows. No idea if thereās anything like this for Linux.