Remove the striker by unscrewing it and pulling it out. Reverse the removal procedure to install a new doorknob. Be sure to orient the new striker in the correct direction or the door won't close properly. In most cases the holes for the new doorknob will match your existing holes. Depending on your door, you may need to adjust the backset, the distance between the edge of the door and the center of the doorknob.
Most doorknobs can accommodate different backsets.
If you encounter resistance when you try to slide the new knob together, turn the lock back and forth so that the shaft fits properly. Don't use cheap doorknobs. In addition to protecting your home, doorknobs also make an impression on visitors.