updating nested properties based on an action property

redux question

//reducer.js

initState = {
	foo: {
		value_a: "",
		value_b: ""
	},
	bar: {
		value_a: "",
		value_b: ""
	}
}

export default(state = initState, action) => {
	switch (action.type) {
		case UPDATE_VALUE:
			return {...state, <WHAT TO DO HERE :(>}
	}
}


//actions.js
//example call: updateValue('foo','value_a',"wohoo");

export const updateValue = (foo_or_bar, property_to_update, value) => {
	type: "UPDATE_VALUE",
	foo_or_bar, <which parent key to update>
	property_to_update <updating value a or b>
	value <value to assign to value_a or value_b>
}