Hi @780bambam.
Open the Fitbit app on your phone - tap on the heart rate tile - tap the gear icon in the top right corner - under exercise tap heart health - check the box next to custom max heart rate. You should be able to set you max heart rate value here to a lower value, which will also affect your zones (lower them). You may have to play around with the value a bit until you think the zones reflect closely to what you believe your zones are.
Once you've done this you should see that the zones are lower in the active zone minutes tile, and that you should also be able to earn them easier.