You can set up the zones anyway you like in say, Garmin Connect, and then download to your device. The first step is to establish your HRmax, if you run as well as ride be aware that your HRmax for running will differ to your HRmax for cycling. Then work out your zones as percentages of your HRmax.
Usually, the fitter you are, the higher your aerobic threshold (rate you can sustain long term) as a percentage of your maximum. But the shift is not huge, from about 85% of max to about 92% ultimately. Zone 6 is usually set from 92% to 100%, so by definition it's impossible to be in Zone 6 for more than a minute or so at a time.