remove the layer of the cap you wish to take the cap off of and then take something small (a small screwdriver works well flat head) and push just under the clip and push the cap off
I really annoys me that you can't pop of the center caps without taking the cube apart. I like to change the tension by making fine adjustments, putting the center caps back on, playing with it for a bit and then adjusting again.
You basically have to disassemble the entire cube, then place a finger on each notch and push them in and up. But seriously Haiyan cube is lame anyway.