Rear diff oil change/check

Hey how are y'all doing, I was going to check my rear diff fluid an possibly change it but I'm having a problem. I have watched videos on how to check an change fluid but the videos show there a plug you pull but I don't see a plug in mines I did see this squared shape bolt in the side ,just trying to make sure this is it before I unbolt something I shouldn't thank you any advice would be great