There are many ways to do this :-

Option A

1. If your house already have LAN cables run through each floors, then use any wifi router to configure it as AP mode.
2. Configure the SSID to be the same name & same password on each router so that Smartphone or Laptop will auto-connect to the nearest unit.


Option B

1. If your house don't have any LAN cables.
2. Purchase TP-Link MESH Wifi devices, each floor with its own unit.
3. Depending on the flooring thickness, you may need to buy extra for the "in between floor".
4. Configure the Master unit and adopt all the Slave units.
5. Special Note, each MESH unit need a power-point so you need to ensure there's an extra plug point available.


Option C

1. Lay new LAN cables to each floor's ceiling.
2. Buy TP-Link Omada series AP for each floors with PoE Adapter and mount it to each floor's ceiling.
3. All APs are to connect directly to Router.
4. Main router Wifi must be off to avoid conflicts.
5. Optional but recommended to also installed Omada SDN Application on any of the OLD/UNUSED PC, so you can have a centralized, features rich configuration & overview of your wifi network.


Hope this helps...