Q: How can I increase wifi bandwidth between two floors?

Use wires, its significantly better then wireless (which will reduce bandwidth available)

Some reading here:

Wi-Fi base stations: Extending the range of your wireless network by adding additional Wi-Fi base stations - Apple Suppo…

The best way to do this.....call it Method 1...... is locate an AirPort Express in the middle of each area where you need more wireless coverage, and connect it back to the AirPort Extreme using a permanent, wired Ethernet cable connection.

The reason for this is that there is no signal loss through an Ethernet cable up to about 330 feet, or 100 meters, so each AirPort Express will receive a full bandwidth signal to broadcast in its own area. This is the best way by far to extend a network.

The AirPort Express can connect to the AirPort Express using a wireless signal.....call this Method 2.....and "extend" the network that way, but there will be a significant loss of about half of the potential speed on the entire network when you do things this way.  And.....the AirPort Express will need to be located about half way between the AirPort Extreme and the area where more wireless coverage is desired.

Things will get tricky with two AirPort Expresses if you try to extend using Method 2, because both Expresses will need to be located about the same distance from the AirPort Extreme.....AND.....both Expresses will need to be located closer to the AirPort Extreme than they are to each other.  The reason for this is each Express must connect directly to the AirPort Extreme.....not to another Express that might already be extending the network.

As you might imagine, there will additional loss of entire network speed with two Expresses, so it may be better to use only one Express.

I think that you can easily understand why Method 1 is the way to go if you want the best performance and best reliability from your network. Hopefully, this is the method that you will choose. Post back if you need a few more tips on the setup.

I outlined the wireless strategy in Method 2 of my post above.

But, if you cannot run the Ethernet cables, please think about using Ethernet over Powerline (EOP) adapters to send the Ethernet signal over the AC powerlines in your home.  This type of connection would be far superior to a wireless connection.....which is the last choice that you want to use when extending a network.

Even if you have line-of-sight between AirPorts when you try to extend using wireless, the speed of the entire network will slow down by about half. If the wireless signal must pass through a ceiling, then you should expect to lose another 20-30% of performance.

I have to agree with Bob wirelessly extending a network is the worst way possible.

I didn't find this information in my initial search on Apple website.  Very helpful.

Unfortunately, the illustrations and "how to" instructions in this document are more than 4 years out of date. Much has changed.

These devices have been around for the last 10 years or so. They are much better now than just a few years ago. The theory with these devices is that you already have the wiring in place......you just need the adapters.

Any computer / electronics superstore will have a selection of adapters from which to choose.

Nice work!  Glad that you are getting the performance that you deserve.