this old video for a older model of the heatsink you have will illustrate you about the plastic lock, it is different but the movement is the same, if you are using the one that came with the cpu in the box that thing is strong but if you have doubts and know how to handle a screwdriver you can remove it by hand but you also can break it or break mainboard
the model you have doesn't have two screws, the black plastic part should be what holds the heatsink in place but after you do the same movement done in the video both sides should be released
the twisting heatsink movement that didn't worked for me but you see used here, do it
after you remove the heatsink you will have to use thermal paste or buy a new thermal compound to replace the one it has now, you can't reuse it, could be dangerous, in two years will stop working and system will become hotter and hotter