_follow_btn.html.erb

<div id="follow-btn">
  <% if current_user.is_following?(user) %>
    <%= link_to user_unfollow_path(user.username), method: :delete,
                                                   data: {
                                                     action: 'ajax:success->follow#update'
                                                    },
                                                   remote: true do
    %>
      <button class="btn btn-blue ml-4">Unfollow</button>
    <% end %>
  <% else %>
    <%= link_to user_follow_path(user.username), method: :post,
                                                 data: {
                                                   action: 'ajax:success->follow#update'
                                                 },
                                                 remote: true do
    %>
      <button class="btn btn-blue ml-4">Follow</button>
    <% end %>
  <% end %>
</div>