I have a Holley 65-84021 carb, dual feed/single pump. I was just wondering how to replace the jets in my carb. I have added headers to my car, so I am thinking that it might be a good idea to learn how to rejet my carb. As I have never had to do this, I figured that this would be the best place to find out how. Any info would be appreciated. A pic of the carb...

You do not nessessarily need to rejet your holley carb because you added headers.

On that style of carb you have to remove the top of the carb
to get at the jets. You'll have to remove the choke linkage on the side of the carb to get the top off. The jets them can be unscrewed with a flat blade screwdriver and replaced.

But as long as you are not getting any lean surge while driveing, the out of the box jetting is probabily fine.
So don't get too carried away.