[q] how to underclocked cpu clockspeed?

Hi Guys I just want to ask to how to underclock the cpu of my phone specifically it runs on Snapdragon 810 im not new to ubuntu.Im working on a custom rom that will fix my xperia z4 to a better use.. Pls help me i would like to underclock it from 2.5 to 1.3 . Thanks and Godblesa!