map - Using Clojure update-in with multiple keys -

I am trying to apply the function to all the elements in a map that matches a certain key.

  (def mymap {: a "a": b "b": c "c"}) (Update-in Mymop [: a: b] # (str "X -"% ))   
 I hope  
  {: a "xa", c "c",: b "xb"}   
 But I get  
 
 Class class expansion in java.lang.String in clojure.lang.Angociative clojure.lang.RT.assoc (RT.java:702) Can not be cast   
 Can anyone help me with this?   
 
 
  update-in  to update a key in the map ( At the level of a particular nest,  [: a: b]  inside the map value of the key: key: a.  
 Can be:  
  (# commode% 1% 2 (str "X-" (% 1% 2)) mymap [: a: b])