A swap is not a new install. The old unit is already connected, the plumbing has already been set around it, and the house has been living with it. The sequence below is what a careful crew follows, and it is also a list you can check the crew against while the work is underway.
Before the crew arrives
Confirm which unit is coming out and which is going in, and get the new model number in writing. Find the main shut-off valve and the valve that sends water around the treatment unit, and make sure both turn freely. Ask the provider what they need from you, such as clear access to the tank area, a working drain, and an outlet if the new unit needs power. Confirm whether the work needs a permit under your local rules and who pulls it.
Shut off and relieve the pressure
Set the house to bypass the old unit before the supply is shut off, so the house keeps water while the unit is out. Shut off the supply to the unit, then open a faucet downstream to relieve the pressure trapped in the lines. Water that stays under pressure can spray when a fitting is loosened, so this step should happen before anything is disconnected.
Remove the old unit and check the connections
Disconnect the old unit's inlet, outlet, and drain connections, and note how each one was connected. Inspect the fittings and the valves for corrosion, cracks, or worn seals. A worn valve found now is much easier to replace while the line is open than after the new unit is running. Check that the drain point is still clear and that the floor below the unit is dry.
Set the new unit and connect it
Set the new unit on a level, stable surface and connect the inlet, outlet, and drain to match the layout on the spec sheet. Check that the connections are tight and that no pipe is stressed by the weight or position of the unit. If the unit needs power, confirm the outlet is correct and that the work follows your local electrical rules. Use a licensed electrician where those rules require one.
Start up, flush, and test
Open the supply slowly, check every connection for leaks, and run water through the unit as the manufacturer directs before putting it fully into service. Set the programming, flow, or regeneration settings to match the model and your water conditions. Then move the bypass back so treated water reaches the taps, and test a faucet at the kitchen and one farther away to confirm the water comes through as expected.
Hand over the records
Ask for the model and serial numbers, the installation date, the settings used, the warranty papers, and the replacement schedule for consumables, and keep them with the house papers. Record where the shut-off and bypass valves are and how to operate them. That record is what the next person will need when the unit needs service.