Selecting Wifi

It may be possible, but it's important to understand that the higher 5 GHz signal will have more difficulty traversing through walls/floors/ceilings, etc. so that your phone may not get a quality signal on that band unless it is in the same room. This is true for any wireless client, not just smart phones.

There are potentially two ways to "force" your phone to connect to a particular network:

1. Move the phone to the location of the wireless access point providing the 5 GHz network, and then, cycle the phone's Airplane mode off/on.
2. If your wireless router allows you to rename each wireless radio's network, then do so. Now configure your phone to connect to the 5 GHz one.

Unfortunately, you can't actually force it to so so automatically.

Instead, you would need to connect your phone to the 5 GHz network, and then, turn off the "Ask to Join Networks" option so that it doesn't automatically connect to any previous network.

One potential option is to delete all saved networks, by using: Settings > General > Reset > Reset Network Settings. Then connect to the 5 GHz network.