To set the DDR memory frequency to your desired value open BIOS menu by booting the device and pressing the 'DEL' key repettitively until BIOS menu opens.
Go to:
Advanced ► AMD CBS ► UMC Common Options ► DDR Options ► DDR Timing Configuration
Here, in the lowest part of the page you will have to accept the risks of changing the DDR frequency timings outside of manufacturers specifications.
Set the DDR timing option to manual and set your desired frequency.