Admin editing other user details - Rails 4.1 + latest Devise + latest Simple Form

Hi everyone, I am writing my first Ruby project and I've dived right in the deep end, so apologies if my lack of deep Rails understanding is showing through.

I am trying to create a user management system, so admins can edit the details of users in the system without being logged in as them. I understand Devise does not have such functionality built in and that's what I'm trying to hack together myself.

I am nearly there. I have a 'Manage Users' page that successfully lists users in the system along with an 'Edit User' link next to each one:

app/views/admin/manage_users.html.erb

<h2>Manage Users</h2>
<p>As an administrator, you can add new users to the system. You can also edit or delete existing ones.</p>
<p>
    <% @users = User.all %>
    <ul class="users">
        <% @users.each do |user| %>
    <li><%= user.id %> <%= user.first_name %> <%= user.second_name %> <%= user.email %> <%= link_to "Edit User", edit_user_details_path(id: user.id) %></li>
        <% end %>
    </ul>
</p>

app/views/admin/edit_user_details.html.erb

Clicking on 'Edit User' correctly renders an 'Edit User Details' page according to the user selected (using id to identify):

<h2>Edit user details</h2>
<p>
    <%= @user.first_name %>
    <%= @user.second_name %>
    <%= @user.email %>
</p>

I now want to implement a form (using simple form) on the page shown above that allows the admin to make changes to the user's first name, second name, email, and an option to reset password. It should ideally behave like Devise's built in 'edit registrations' page but change a user who is not currently linked in. I'm completely stumped as to how I will achieve this. Any help appreciated.

Here is my Admin controller file for context:

app/controllers/admin_controller.rb

class AdminController < ApplicationController
  def manage_users
  end

  def edit_user_details
    @user = User.find(params[:id])
  end
end