If you got the simple rear, it should have a side-to-side dovetail groove. It would replace your rear sight.
First, put the rifle/barrel in a padded bench vise...this keeps it from moving while you are working on it & helps to avoid accidental damage.
You simply use either a screw-type sight tool, or a hammer & brass/nylon punch, and gently/not-so-gently push it out from left to right.
That's left to right from the REAR of the rifle looking towards the barrel.
I was taught to remove dovetails from left to right as the barrel faces away from you. Which means tap out to the right.
Reason being that while tapered female dovetails may or may not still be used by some manufacturers, male dovetails that are tapered are still very common.
Installation is right to Left, of course
If you have anything else, we need to see a pic...
Oh, the front screw locks the sight on the dovetail....
The middle adjusts the height...
and the rear-most screw in the center of the U-shape locks the side-to-side adjustment.
And for those seeking a regular (non-fibre-optic) target rear sight...this is your $10 huckleberry Rear Sight, Adjustable, Marlin Factory
Same adjustments as above.