Elektra4Houston

I noticed a change in the jfile
https://mistresselektra.com/wp-admin/post.php?post=199&action=edit#

I am using a siteground_59 template (revinformatics.com) …would anyone have expertise in how to replace the main template with a slideshow?
Here are its menu choices:
Templates
404 Template
(404.php)
Archives
(archive.php)
archives.php
Attachment Template
(attachment.php)
Popup Comments
(comments-popup.php)
Comments
(comments.php)
Footer
(footer.php)
Theme Functions
(functions.php)
Header
(header.php)
header-img.php
Main Index Template
(index.php)
links.php
Page Template
(page.php)
Search Results
(search.php)
Search Form
(searchform.php)
Sidebar
(sidebar.php)
Single Post
(single.php)
templates.php
Styles
Stylesheet
(style.css)
RTL Stylesheet
(rtl.css)

Hello
I understand about replacing images in the URL’s, etc and the settings in the admin area…
I am using a custom template…siteground _ 59
and cannot see its header change
Where do I go to point the TEMPLATE header to the new XML header?
In other words…where do I go to replace the template header with my new header?

My site is revinformatics.com

Thank you
Carolyn