 Two likely issues:  if you could not find the higher refresh screen with a new system, you will likely face the same challenge finding one from anywhere else;  this is a current model, and Dell is likely the only source of parts. The other issue is that replacing the screen will void your warranty, if that's a consideration.  
I could only find the 120hz version of the Ryzen G15 in my region. I was wondering if I could replace the screen as the response times and colours are subpar compared to other gaming laptops?
Replacing a 120hz G15 screen with a 165hz one?