change username param to _username

app/controllers/users_controller.rb

@@ -28,7 +28,7 @@ def create
   def update
     if @user.nil?
       render json: { errors: 'User not found' }, status: :not_found
-    elsif @user.update(user_params)
+    elsif !@user.update(user_params)
       render json: { errors: @user.errors.full_messages },
              status: :unprocessable_entity
     end
@@ -45,7 +45,7 @@ def destroy
   private
 
   def find_user
-    @user = User.find_by_username(params[:username])
+    @user = User.find_by_username(params[:_username])
   end
 
   def user_params

config/routes.rb

@@ -1,6 +1,6 @@
 Rails.application.routes.draw do
   # For details on the DSL available within this file, see http://guides.rubyonrails.org/routing.html
-  resources :users, param: :username
+  resources :users, param: :_username
   post '/auth/login', to: 'authentication#login'
   get '/*a', to: 'application#not_found'
 end