The most used method for beginners, using a SSL fixer plugin. if you are web developer you may fix it by editing your code.

Simply install and activate the Really Simple SSL plugin, Upon activation, you need to visit Settings » SSL page to review plugin settings. Really Simple SSL works out of the box and will automatically take care of SSL / HTTPS settings and fix mixed content errors.

Note: The plugin attempts to fix mixed content errors by using output buffering technique. This can have a negative performance impact on your site. However, if you are using a caching plugin, then it will only impact the first page load.