change the WiFi speed to 2.4 on an iPhone 13 Pro Max

Are you perhaps trying to set up a device that only uses 2.4? I have a ring camera, as well as a thermostat, that are only 2.4. I had to go into my router and temporarily shut off the 5GHz channel. This forces the phone to use 2.4, since that is the only channel working. After setting up the device, turn your 5 GHz back on and everything will work as designed.

Alternatively, you can set up a guest network on your router that is only 2.4 and use that network for your devices.

First, check the settings on your Wi-Fi router to confirm that the 2.4 GHz and 5 Hz networks have separate names, also known as SSIDs.

Be sure to select the 2.4 GHz network from your iPhone.

If your Wi-Fi router is using the same name or SSID for both the 2.4 GHz and 5 GHz signals, there is no way to force your iPhone to connect to the 2.4 GHz (or 5 GHz) network. The phone will connect to the signal with the best combination of signal strength and speed.

Sorry, your question does not make sense as written.

I assume you're trying to ask how to force your Mac to connect to a 2.4GHz network.

Ideally, if you have both a 2.4 GHz and 5GHz network available, they should have different SSIDs. Join the correct network. You cannot force your machine to join a 2.4GHz network if both the 5 and 2.4GHz networks have the same SSID and password.