No, there’s no special configuration for 5GHz other than the usual BlinkUp. You just use the Electric Imp app to select a 5GHz network, enter the password and do BlinkUp, and the imp005 will connect.
Do your 5GHz and 2.4GHz networks have the same name? If so, the imp005 will probably connect to the 2.4GHz radio first (I don’t know this for a fact, but most of the various devices I have encountered do).
FWIW, I keep my 2.4GHz and 5GHz networks separate for this very reason.
If your networks do not use the same name, you should note that you can only store one network on the imp, so if you BlinkUp with the 5GHz network and then immediately re-Blink with the 2.4GHz network, the imp will only remember the second one.