Clojure nested map - change value

have to say I started learning Clojure about two weeks ago and now I'm stuck on a problem since three full days.

I got a map like this:

{
  :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"}
}

and need to change :team "X" to :team "H". I tried with a lot of stuff like assoc, update-in etc. but nothing works.

How can I do my stuff? Thank you so much!