Не так много информации, но ... попробуйте:
var additionalProps = {}
data.forEach(obj => {
geoid = obj.STATE_FIP + obj.COUNTY_FIP
additionalProps[geoid] = obj
})
var updatedGeoJson = {
"type": "FeatureCollection",
"features": []
};
oldGeoJson.features.forEach(x => {
geoid = x.properties.GEOID
newProps = {...x.properties, ...additionalProps[geoid]}
x.properties = newProps
updatedGeoJson.features.push(x)
})