If the item being embedded is located within the same website (domain), then you should update the URL to just include the path to the item (e.g. /example.png) without a scheme and hostname (e.g. do not include http://www.example.com).
If the item is located on a different website, then you should use a full URL that starts with "https://" (e.g. https://www.example.com/example.png).
If you do not know which items are being embedded incorrectly, then we recommend that you search the HTML source of the page for "http://". Please note that you only need to update the paths for embedded content, not links.
Below are some examples of the warning that you might receive in different browsers if there are items that are not embedded securely.