This is a common problem when moving a website over from HTTP to HTTPS. In general, the various resource URLs (like images, videos, fonts, etc.) that you have in your content and are stored in the database need to be replaced.
This can be done in a number of ways. The easiest would be to use a plugin such as:
This is a very popular plugin that is used to automatically find and replace a specific keyword in your database.
Note: Before making any changes to your database, it's always recommended to create a full backup. You can follow this guide to know how you can take a backup of the site.
Once you've installed the plugin, the process is straightforward.
Search for: http://yourdomain.com/
Replace with: https://yourdomain.com/
Note: The "dry run" option can be checked if you want to see the changes before they are applied.