My theme contains a parallax header image. How can I change the URL of the header image so that it uses HTTPS? I want to do it manually.

Currently the URL is:

http://example.com/themes/abc/header.jpg

I want to change it to:

https://example.com/themes/abc/header.jpg

Plugins are not helping me on this specific file (they are converting others). Chrome tells me the header is being loaded by jquery.js. I have no idea how jQuery is doing this.

I believe editing functions.php can help, but how can I do it?