Hello, lizhenmiao 
You can find information regarding this here: https://meta.discourse.org/t/set-up-file-and-image-uploads-to-s3/7229#toc-h3-ive-enabled-s3-uploads-in-my-discourse-instance-which-has-been-going-for-a-while-what-do-i-do-with-the-existing-local-uploads