Some water treatment tasks suit a careful homeowner, and others need a trained technician with the right tools and, in many places, a permit. The useful question is not whether you are handy. It is which specific task you are about to do and what happens if it goes wrong.
Tasks that are usually fine to do yourself
Swapping the cartridge in a pitcher or a faucet-mounted filter, following the manufacturer's steps, is the main example. So is rinsing a new cartridge as directed, wiping the housing, and noting each change in a log. Keep the manufacturer's instructions with the filter, and use only the replacement part those instructions name.
Tasks to hand to a technician
Cutting into supply lines, changing a bypass, connecting drain or outlet lines for a larger system, or working near a water heater all belong with a licensed hand. So does anything that needs a permit or code sign-off. If a job needs tools you do not own or a shutoff that affects the whole house, it is a technician's job.
Signs a filter has stopped doing its job
A pitcher or faucet filter that has passed its labelled service life may still look fine. Watch for taste or odor coming back, a slow flow that was not there before, a crack or seep at the housing, or water that looks cloudy after it has run. Do not wait for a visual cue. Replace on the labelled schedule and keep track of it.
How to decide case by case
Before any job, write down three things: what the step changes, what it touches, and how you would undo it. If you cannot answer all three from the manufacturer's paperwork, the step belongs with the technician. Ask the technician which steps they would have you do yourself on your system and which they want to keep, so you know where the line sits for your equipment.
When you choose who to hire for the harder jobs, the award winners list for greater St. Louis gives you a set of companies to compare, and you can ask each one which tasks it leaves to the homeowner.