app/views/employees/_change_manager.html.erb
<%= modal title: 'Change Manager', id: 'modal', button: {class: :en} do %>
<div class="modal-body">
<%= simple_form_for employee,url: change_manager_employee_path(employee) do |f| %>
<%= f.input :manager_id,collection: @employees %>
<%= f.button :submit %>
<% end %>
</div>
<div class="modal-footer">
<button type="button" class="btn btn-primary">Change</button>
</div>
<% end %>