Lever doorknobs are horizontal brass or pewter bars that you press up or down to open a door, rather than turning a round knob. They may look vintage, but they install just like regular modern doorknob-and-latch assemblies, while making sure the lever is pointing in the correct direction. It's a project that a competent homeowner can do in less than an hour.
Open the door. Remove the old knob assembly by taking out the two screws on the collar of the interior doorknob. Pull the doorknobs straight off on both sides.
Move to the narrow front edge of the door. Remove the latch-plate screws on the edge and pull the latch assembly straight out.
Slide your new latch assembly into the opening at the narrow edge of the door, where the old assembly was. Make certain that the bevelled side of the latch is facing the doorway. Secure the latch with the provided screws.
Slide the exterior portion of the door-handle assembly through the hole in the front of the door. Make sure the lever is pointing away from the front edge of the door. Guide the spindle and other parts of the handle assembly through the appropriate holes in the latch assembly, which will be visible through the hole in the front of the door.
Set the interior portion of the door assembly into the hole from the interior, again making sure the lever is pointing away from the front edge. Guide the interior assembly onto the spindle from the exterior assembly. Slide the two long doorknob screws into the two holes at the base of the interior lever, and thread them into the receiving holes of the exterior assembly.
Tighten the two doorknob screws so that both the interior and exterior knobs pull up tightly to the face of the door on each side.