Clojure nested map - change value

The walking functions are good for replacement like that.

(clojure.walk/prewalk-replace {[:team "X"] [:team "H"]} map)

Passing in vectors allows you to ensure that you don't just replace all the "X"s.

assoc-in is used for replacing or inserting values in the map specified by path

(def m { :agent1 {:name "Doe" :firstname "John" :state "a" :time "VZ" :team "X"}
         :agent2 {:name "Don" :firstname "Silver" :state "a" :time "VZ" :team "X"}
         :agent3 {:name "Kim" :firstname "Test" :state "B" :time "ZZ" :team "G"}})

(assoc-in m [:agent1 :team] "H")

{:agent1 {:state "a", :team "H", :name "Doe", :firstname "John", :time "VZ"},
 :agent2 {:state "a", :team "X", :name "Don", :firstname "Silver", :time "VZ"},
 :agent3 {:state "B", :team "G", :name "Kim", :firstname "Test", :time "ZZ"}}

however, if you want to update ALL team "X"'s, regardless of specific path, over all recursive levels of the tree, you can use clojure.walk's prewalk or postwalk functions combined with a function of your own:

(use 'clojure.walk)
(defn postwalk-mapentry
    [smap nmap form]
    (postwalk (fn [x] (if (= smap x) nmap x)) form))

(postwalk-mapentry [:team "X"] [:team "T"] m)

{:agent1 {:state "a", :team "T", :name "Doe", :firstname "John", :time "VZ"},
 :agent2 {:state "a", :team "T", :name "Don", :firstname "Silver", :time "VZ"},
 :agent3 {:state "B", :team "G", :name "Kim", :firstname "Test", :time "ZZ"}}