If you are using the hard drive that came with the computer, you should have the software already installed from ASRock. Look for the "ASRock Utility" folder and the program is called "ASRRGBLED".
If you are NOT using the included hard drive, you need to do a little more work. I had to search for my specific motherboard on ASRock's website: https://www.asrock.com/mb/index.asp. click on 'Support' and then 'Downloads' and download the ASRock Polychrome RGB ver:1.0.42 file. I initially downloaded the wrong version by not selecting the correct motherboard and I was not able to get the software to run until I removed the incorrect version and installed the correct one corresponding to my motherboard. My build came with the Z390 Phantam Gaming 4 motherboard and here is the link to the downloads: https://www.asrock.com/mb/Intel/Z390%20Phantom%20Gaming%204/index.asp#Download.
If you need help finding out what motherboard you have and you are running windows 10, press the WINDOWS KEY + R, then type msinfo32 and press Enter to bring up the System Information utility.
I really wish iBuyPower mentioned something about this or had some instructions. The very first thing I did when I bought my computer was swap out the hard drive and I haven't reconnected the one included yet because I don't have the SATA cable I need to do so (It needs an L shaped cable to avoid hitting other components). I had to come online and after a little bit of searching found out what I needed to download to change the RGB LED's.