Login to your WordPress Admin area, usually accessible at http://example.com/wp-admin (where example.com is your domain name). Click on Add New under the Plugins menu. Search for Better Search Replace at the top right. Then click the Install Now button on the Better Search Replace plugin. After it installs, click the Activate button. After activating, click Better Search Replace under the Tools menu.
Under Search for put in your web site URL (for example: http://example.com or http://www.example.com). Under Replace with put the same URL with https at the beginning (for example: https://example.com or https://www.example.com). Under Select tables highlight all tables by clicking on a table name, then pressing Ctrl-A on Windows or Cmd-A on Mac). Uncheck the Run as dry run? checkbox. Then click Run Search/Replace.
If you would like, you can now remove the Better Search Replace plugin by clicking on Installed Plugins under the Plugins menu. Then click on Deactivate under the Better Search Replace plugin. Followed by clicking Delete.
Next we recommend following the instructions in our Force HTTPS knowledge base article.