finally, setting the display_name works.

app/controllers/user_controller.rb

@@ -9,9 +9,10 @@ class UserController < ApplicationController
 
   def update
     authenticate_user!
-    params.require(:display_name)
-    current_user.display_name = params[:display_name]
-    current_user.save!
+    user_attributes = params.require(:user).permit(:display_name)
+    unless current_user.update_attributes(user_attributes)
+      flash[:error] = "fork it"
+    end
     redirect_to edit_user_profile_path
   end
 end

app/models/user.rb

@@ -15,7 +15,9 @@ class User < ActiveRecord::Base
   SCREEN_NAME_REGEX = /\A[a-zA-Z0-9_]{1,16}\z/
   
   validates :screen_name, presence: true, format: { with: SCREEN_NAME_REGEX }, uniqueness: { case_sensitive: false }
-  
+
+
+
   def login=(login)
     @login = login
   end

app/views/user/edit.html.haml

@@ -3,7 +3,7 @@
   .col-md-9.col-xs-12.col-sm-9
     .panel.panel-default
       .panel-body
-        = bootstrap_form_for(update_user_profile_path, method: "patch") do |f|
+        = bootstrap_form_for(:user, method: "patch") do |f|
 
           = f.text_field :display_name, label: "Your name"