Nice build by the way 
Since I don't see a cpu cooler in that build, I am assuming you are using the default CPU cooler with the MB. From the installation instructions in the manual here: https://a248.e.akamai.net/f/1828/2290/9f/dlcdnet.asus.com/pub/ASUS/mb/SocketAM3+/M5A99FX_PRO_R20/E8043_M5A99FX_PRO_R20_V3_WEB.pdf (diagrams on pg 58), it looks like you need to pull back the lever on one side and on both sides push in some clips on both sides near the bottom to release the cooler.
Since I don't see a cpu cooler in that build, I am assuming you are using the default CPU cooler with the MB. From the installation instructions in the manual here: https://a248.e.akamai.net/f/1828/2290/9f/dlcdnet.asus.com/pub/ASUS/mb/SocketAM3+/M5A99FX_PRO_R20/E8043_M5A99FX_PRO_R20_V3_WEB.pdf (diagrams on pg 58), it looks like you need to pull back the lever on one side and on both sides push in some clips on both sides near the bottom to release the cooler.