Enter programming mode; 
Hold the DOWN pedal assist button on the left and the LIGHT button at the same time on the left LCD display. Hold for approximately 5 seconds.

Once in programming mode...
Then hold all 3 buttons: Hold the pedal assist UP and DOWN button and the LIGHT button at the same time (hold for about 10 seconds). 

On my Radcity 5 plus this opened a new menu allowing me to specify max power level for the motor and the ability to increase the speed limiter.  I raised mine to 38 or 40 km/hr. 

Yes...I have a Radcity 4 and 5 plus.  On the 5 plus...I find PAS 2 useless (not enough power) and PAS 3 (too much on the flats sometimes).  The PAS levels are not well thought out.  I'd prefer about 220 watts in PAS 2 (like the City 4) and PAS 3 should be about 320 watt (not the 380 watts I see).  Overall I like the Radcity 5 plus...but I find it harder to find a nice cruising PAS level. If I am going for maintaining speeds above 30 km/h...the 5 plus is good.