You can disable 5.0ghz by going to the WIRELESS tab, on the left hand side you go to Network
Now, you can also disable the 5 Ghz radio in one AP. On this case, you need to enter on the AP you want to disable it. The AP needs to be in Custom mode in order you to be allowed to shutdown the radio.
And lastly you can remove the 5.0 Ghz from the WLAN and only propagate in 2.4Ghz