https://www.ebay.ca/itm/113418725892

Want to install this rear diffuser soon. How do you install this? do you need to replace the rear guard of the bumper? or does it just go over stock guard?

 

That looks like a replica of the seibon cf rear diffuser for our cars. It will be a direct replacement of your stock diffuser.

 

Looking at the link the initial thought is that you need to remove the rear bumper and remove the stock diffuser and install this one in it's place.
However, none of the product pics shows the inside the of the diffuser, so it's hard to say if it has mounting points that would allow it to be mounted directly to the bumper.
Also the product comes with self tapping screws...which would suggest that it mounts over the existing diffuser, so there's no need to take anything apart...if that's the case then the inside perimeter of the diffuser should have 3M tape to stick it down and the screws are there as additional security.
Unless someone has installed this exact part it's going to be hard to say for sure how it goes on.
Stillen made a stick-on diffuser...so maybe it's like that one.