How do you change MongoDB user permissions?

If you want to just update Role of User. You can do in the following way

db.updateUser( "userName",
               {

                 roles : [
                           { role : "dbAdmin", db : "dbName"  },
                           { role : "readWrite", db : "dbName"  }
                         ]
                }
             )

Note:- This will override only roles for that user.