(def person {:name "Vesa Marttila"
             :address {:street "Pursimiehenkatu 21"}})

(update-in person
           [:address]
           merge {:postal-code "00150" :city "Helsinki"})
; -> {:name "Vesa Marttila", :address {:city "Helsinki", :postal-code "00150", :street "Pursimiehenkatu 13"}}