Lever handles are one of the simplest changes you can make to an interior door, and one of the easiest to get slightly wrong. A handle that rattles, a latch that does not seat, or a spindle that is too long will work for a day and then start to annoy everyone who uses the room. Take the job in order, and test each step before you move on.
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Before you buy anything
Take the old handle off the door first, or at least look closely at it. Many interior doors use a round bore, and the distance from the edge of the door to the center of that bore is called the backset. Many houses have a backset of 2 3/8 inches or 2 3/4 inches, so measure yours rather than guessing. Check the door thickness range printed on the package, and check whether the set is a passage handle, which has no lock, or a privacy handle, which has an inside button and an outside release hole. Buy the set that matches what the door already does.
Remove the old set
Remove the screws holding the inside and outside knobs, pull the knobs free, and keep the screws together in a small dish. Next, remove the latch from the edge of the door. Some latches slide out once the screws are out, and others need a gentle push. If something will not move, stop and look for the screw you missed rather than forcing it, because a forced latch can split the edge of the door.
Install the latch
Slide the new latch into the bore in the door edge so the faceplate sits flush. Most latches have an arrow on the faceplate that shows which way the latch bolt should face, so follow the instructions for your set. Run the spindle through the latch so the handle turns the bolt cleanly in both directions. A latch that binds here will bind later, so fix it now.
Set the strike plate and the trim
Close the door and see exactly where the latch bolt hits the frame. If the strike plate does not line up, mark its position, cut a shallow recess if the plate needs one, and tighten it so the bolt drops in without catching. Then fit the inside and outside handles over the spindle, line them up, and tighten the screws evenly. Do not overtighten, because the handle can crack the trim or strip the screw holes.
Test it before you call it done
Work the lever from both sides. The latch should retract fully and return without sticking. Close the door gently and check that it latches on its own and holds when you push. On a privacy set, test the outside release with the small tool that came with it, and confirm the door opens from outside every time. Do this test with the door fully closed and then open, not only with the door standing ajar.
When to stop and call someone
Stop if the door is an exterior door, a fire-rated door, a pocket door, or if the bore hole is split, badly off-center, or in a door edge too thin for the latch. A carpenter or locksmith can fix those without damaging the door. Also stop and talk with the family before you install a lock on any room she uses, especially one she might be alone in at night. Decide together who holds a key or release tool, and whether she should be able to lock the door at all. That decision is a family and care-team question, not something the hardware package can answer. If she has mentioned that her grip or her hand strength has changed, bring that to her care team as well, because a handle choice should not be the only answer to a change she has noticed.