_form.html.erb

<%= form_for(@user, :url => { :action => "update_password", :controller =>"users", method: "patch" }) do |f|  %>
  <div class="field">
    <%= f.label :password, "Password" %><br />
    <%= f.password_field :password, :autocomplete => "off"  %>
  </div>
  <div class="field">
    <%= f.label :password_confirmation %><br />
    <%= f.password_field :password_confirmation %>
  </div>
  <div class="action_container">
    <%= f.submit %>
  </div>
<% end %>

log.rb

Started GET "/user/update_password" for ::1 at 2017-03-30 10:19:59 -0400

ActionController::RoutingError (No route matches [GET] "/user/update_password"):
  actionpack (4.2.6) lib/action_dispatch/middleware/debug_exceptions.rb:21:in `call'
  web-console (2.3.0) lib/web_console/middleware.rb:28:in `block in call'
  web-console (2.3.0) lib/web_console/middleware.rb:18:in `catch'
  web-console (2.3.0) lib/web_console/middleware.rb:18:in `call'
  actionpack (4.2.6) lib/action_dispatch/middleware/show_exceptions.rb:30:in `call'
  railties (4.2.6) lib/rails/rack/logger.rb:38:in `call_app'
  railties (4.2.6) lib/rails/rack/logger.rb:20:in `block in call'
  activesupport (4.2.6) lib/active_support/tagged_logging.rb:68:in `block in tagged'
  activesupport (4.2.6) lib/active_support/tagged_logging.rb:26:in `tagged'
  activesupport (4.2.6) lib/active_support/tagged_logging.rb:68:in `tagged'
  railties (4.2.6) lib/rails/rack/logger.rb:20:in `call'
  actionpack (4.2.6) lib/action_dispatch/middleware/request_id.rb:21:in `call'
  rack (1.6.5) lib/rack/methodoverride.rb:22:in `call'
  rack (1.6.5) lib/rack/runtime.rb:18:in `call'
  activesupport (4.2.6) lib/active_support/cache/strategy/local_cache_middleware.rb:28:in `call'
  rack (1.6.5) lib/rack/lock.rb:17:in `call'
  actionpack (4.2.6) lib/action_dispatch/middleware/static.rb:120:in `call'
  rack (1.6.5) lib/rack/sendfile.rb:113:in `call'
  railties (4.2.6) lib/rails/engine.rb:518:in `call'
  railties (4.2.6) lib/rails/application.rb:165:in `call'
  rack (1.6.5) lib/rack/lock.rb:17:in `call'
  rack (1.6.5) lib/rack/content_length.rb:15:in `call'
  rack (1.6.5) lib/rack/handler/webrick.rb:88:in `service'
  /Users/juan/.rbenv/versions/2.3.0/lib/ruby/2.3.0/webrick/httpserver.rb:140:in `service'
  /Users/juan/.rbenv/versions/2.3.0/lib/ruby/2.3.0/webrick/httpserver.rb:96:in `run'
  /Users/juan/.rbenv/versions/2.3.0/lib/ruby/2.3.0/webrick/server.rb:296:in `block in start_thread'

routes.rb

Rails.application.routes.draw do
  resources :tasks


  devise_for :users


  resource :user, only: [:edit] do
    member do
      patch 'update_password'
    end
  end


  root 'tasks#index'

end

users_controller.rb

class UsersController < ApplicationController

  before_action :authenticate_user!

  def edit
    @user = current_user
  end

  def update_password
    @user = User.find(current_user.id)
    if @user.update(user_params)
      # Sign in the user by passing validation in case their password changed
      bypass_sign_in(@user)
      redirect_to root_path
    else
      render "edit"
    end
  end

  private

  def user_params
    # NOTE: Using `strong_parameters` gem
    params.require(:user).permit(:password, :password_confirmation)
  end
  
end