How to Properly Move WordPress from HTTP to HTTPS

Setup SSL/HTTPS in WordPress Using a Plugin

First, you need to install and activate the Really Simple SSL plugin.
Upon activation, you need to visit Settings » SSL page. The plugin will automatically detect your SSL certificate, and it will set up your WordPress site to use HTTPs.
The plugin will take care of everything including the mixed content errors. Here’s what the plugin does behind the scenes:

• Check SSL certificate
• Set WordPress to use https in URLs
• Set up redirects from HTTP to HTTPs
• Look for URLs in your content still loading from insecure HTTP sources and attempt to fix them.

Note: The plugin attempts to fix mixed content errors by using output buffering technique. It can have a negative performance impact because it’s replacing content on the site as the page is being loaded. 
This impact is only seen on first-page load, and it should be minimal if you are using a caching plugin.
While the plugin says you can keep SSL and safely deactivate the plugin, it’s not 100% true. You will have to leave the plugin active at all times because deactivating the plugin will bring back mixed content errors.