Having visitors reporting they received mixed content warnings is annoying (especially when your site is SSL protected).
What can you do? Try Really Simple SSL Plugin. This plugin automate discovery and resolution, a perfect fix for your problem.
To install plugin:
Step 1: Go to WordPress Administration Dashboard. Click on Plugins>Add New>Search for Really Simple SSL>Install Now. Once installed, click ACTIVATE.
Step 2: Backup your site before enabling SSL by following the guide on screen. Once it's done, click Go Ahead, Activate SSL.
Step 3: Proceed to login to WordPress site. Click Settings>SSL. Review settings of Really Simple SSL and ensure that Mixed content fixed was successfully detected on the front-end is checked as per picture below.
This plugin is unable to locate external URLs that caused the mixed content warning thus manual work needs to be done on this part.