I've never used Model Builder, preferring straight python, but here's a python function to do what you want:
def CalcField(zone):
risk = ''
if zone in ('A', 'AE', 'AH', 'AO'): risk = 'High'
elif zone = 'Whatever': risk = 'Moderate'
elif zone = 'X': risk = 'Low'
else: risk = 'somedefaultvalue'
return risk
You want that last else in there to handle any cases that don't match the previous criteria, especially Null values. Call the function, passing in FLD_ZONE.
There is a CalculateField GP tool that I'd assume is available in Model Builder, but as I said, not familiar with MB, so not sure how that would integrate. Anyway, use what you can from the above.
Here's a good example from ESRI (3rd example, Calculate Ranges).