How to Switch Your WordPress Site from HTTP to HTTPS (with a little help from WP Migrate DB Pro)
Step By Step Guide to Migrate Your Website To HTTPS
In-Depth HTTP to HTTPS Migration Guide for WordPress
Really Simple SSL (WordPress plugin)
Why No Padlock? (a simple tool that will tell you about any insecure items on your SSL page)
SSL Check: scan your website for non-secure content
How to Properly Move WordPress from HTTP to HTTPS (Beginner’s Guide)