If you have a full message display, use the OK button. If you don't have the full message display, use the RESET button on the steering wheel.
Hold down that button for five seconds and keep holding it... start the truck (not sure if you have to still keeping holding it here) and the words engineering mode will show up.
pressing the same button again will advance through the information.

Many messages on these forums say to turn the truck "on"... well, you don't just turn the key to the ON position, you have to start the vehicle to get to this mode.

 

So on my 2012 Lariat Eco, in theory I would be able to get a digital speedometer and other items on my cluster?

Might need to play with it and see.

Is it permanent or does one have to do it every time they start the truck?

 

I just saw a few videos.

Looks like you can't permanently change anything.