excels_controller.rb

class ExcelsController < ApplicationController
 def show
 end

 def create
   file = params[:file]
   # ファイルを開く
   workbook = RubyXL::Parser.parse file.path
   # 編集する
   workbook[0].add_cell 0, 0, 'changed'
   # 編集したファイルを送る
   send_data workbook.stream.string, type: file.content_type, filename: 'modified.xlsx'
 end
end

routes.rb

Rails.application.routes.draw do
  # For details on the DSL available within this file, see http://guides.rubyonrails.org/routing.html
  root to: "excels#show"
  resource :excel, only: [:show, :create]
end

show.html.erb

<%= form_tag({action: :create}, multipart: true) do %>
  <div>
  <%= file_field_tag :file %>
  </div>
  <div>
  <%= submit_tag "Send" %>
  </div>
<% end %>