Adding Updated Post Date PHP Custom Theme

In the template, right after the echo get_the_date(), add

$before = " Modified: " ; // note spaces before and after string
$after = " "; 
the_modified_date( $d, $before, $after, $echo );

See https://codex.wordpress.org/Function_Reference/the_modified_date for date formatting options. One of those examples might be what you want to see.

Of course, you’ll want to use a Child Theme, so that theme updates don’t overwrite your changes. Once you create the Child Theme structure, copy your template into the Child Theme folder, then make the changes.