Unscrew the old brass doorknob from the door, using a Phillips-head screwdriver. Once both screws are out, pull both sides of the doorknob away from the door.
Remove the old door plate by unscrewing it from the door frame with the Phillips-head screwdriver and then pulling it off. This step is optional, as most door plates are interchangeable with different doorknobs.
Position each side of the new doorknob over both sides of the doorknob hole. Screw the pieces together, using the Phillips-head screwdriver and the screws that came with the new doorknob.
Install the door plate that came with your new doorknob --- if you carried out Step 2 --- by placing it over the spot where the old one was located and screwing it in, using the Phillips-head screwdriver and the included screws.