update.html.erb

<%= form_tag('/update', :method => "post") %>

<%= select_tag "kgs_names", options_from_collection_for_select(@user, "kgs_names", "kgs_names"), :multiple => true, :class => "kgs_names" %>
<%= select_tag "division", options_for_select(["Alpha", "Beta I", "Beta II", "Gamma I", "Gamma II", "Gamma III", "Gamma IV", "Delta"]), :class => "division" %>

<br />
<%= submit_tag "Submit" %><br/>

<table style="width:400px">
<% @user.each do |item| %>

	<tr>
	  <td><%= item.kgs_names %>
	  </td>
	  <td>
		<%= item.division %>

		</td>
	</tr>

<% end %>

</table>

user_controller.rb

  def update
    @user = User.all
    new_division = params[:division]
    kgs_names = params[:kgs_names]
    
    for x in kgs_names
      user = User.where(:kgs_names => x)
      user.each do |y|
        y.division = new_division
        y.save
      end
    end 
  end