If there are no settings for RAM speed in the BIOS, then I believe that you simply can't change it. The RAM will work at the minimum of the RAM's SPD profile speed and the top speed of the motherboard.
Are you trying to raise the speed of the RAM that it came with, or did you install 1600 RAM? Can you run CPU-Z and provide the information from both the memory tab and the SPD tab (one per memory module for the SPD tab)?