I found a tool (Intel Extreme Tuning Utility, os win 7 ulti x64) what I can use to change the gpu clock on my laptop (cpu type Intel Core i5-4210U, built in Intel HD Graphics 4400). I marked the slider (belongs to that function) with red on this screen of Intel XTU to avoid any mistake.
I would be happy to build such functionality into my own program. That is enough if my program works at least on my own processor (processor type linked above).
My problem is, that I do not know, how to access gpu clock rate or absolute gpu clock, or whatever really exists behind the scene. Some documentation or any advice in advance will be great.