Somewhere between the third time you open the cabinet under Mom's sink and the fourth time you wonder who put the filter there, the job stops being a filter and becomes a project. This is the one-afternoon version, start to finish, so you can decide whether it belongs to you, your sibling, or a handyman before anybody is standing in a puddle.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Check the cabinet before you touch the filter
Pull everything out from under the sink and look at the floor of the cabinet. Soft wood, a stain line, or a warped panel means water has already been getting somewhere it should not. Fix that first, or the new filter will sit on top of the problem. Then look at the two shutoff valves on the supply lines. If they are old, they may not close all the way, and some start to drip the moment you turn them. Decide now what you will do if that happens. If you are not comfortable with a leak, plan to shut the water off at the main for the job, and find the main before you start.
Find out what kind of filter you have
Under-sink filters come in two common shapes: a canister with a housing you unscrew, and a cartridge that twists out of a head. Take a photo of the whole setup before anything comes off, including which tube is the cold line and which way the arrows point. Read the rating sheet on the filter or the box and write down what it says it is certified to reduce. You are matching that to what your household needs, not to what a neighbor bought. Find the replacement by the model number on the old housing, not by a shelf label that says universal.
Gather what you need, plus the thing people forget
You will need a bucket, a stack of towels, a small pan for drips, the new cartridge, and a flashlight. Bring a wrench, but not a cheater bar. Plastic fittings are meant to go on snug, usually hand-tight plus a small turn, and forcing them is how a good afternoon turns into a bad one. The thing people forget is a spot to set the old cartridge down that is not the kitchen floor.
Do the swap in order
Close the valve, then open the faucet and let the line drain. Set the bucket and towels in place. Release the housing or twist out the old cartridge, and expect a little water. Wipe the housing with a damp cloth if it is dirty. Seat the new cartridge the way the old one sat, following the arrows. Hand-tighten, then give the connection the small turn the instructions list. Open the valve slowly, not all the way at once.
Flush it and check every joint
Run the cold tap for as long as the instructions say before you use that water for drinking or cooking. This clears the air and a little loose material from the new filter. Set a dry paper towel under each joint and the housing. Check while the water is running, then again after the house has been using water normally for about an hour. A slow damp spot is the first sign a fitting needs another small turn or a new washer. It is much easier to catch now than at dinner.
Put it on Mom's Week, with one name
This is the part that keeps the swap from falling through. Write the install day on the housing with a marker, and put the next change on the calendar in Mom's row, not the kids' row. Next to it, write one name, the person who gets the reminder and owns the job. If the afternoon helper is already coming in, she can note the reminder and pick up a new cartridge as an errand, but the swap itself stays with a family member or a hired handyman. If this filter keeps falling off everyone's list, the list needs an owner. Nobody did anything wrong.
If the cabinet floor is soft or a valve will not close, stop there and call someone. Local Help in St. Louis is a good place to start for the rest of the house.
You are not behind. You are one cartridge and one name away from a sink that just works.