Search and replace http:// links to https:// to get the green padlock

I’ve experienced this problem in the past. In my opinion, the easiest way is by making use of the plugin: https://wordpress.org/plugins/search-and-replace/

Always perform a backup of your database before performing a search and replace. Make sure you do a “Dry Run” first. If you do not already have good backup procedures in place, check out UpdraftPlus.