By default, the archive page title is simply the name of the category or tag.
If you want to use a custom title instead, try this PHP snippet:
add_filter( 'get_the_archive_title', function( $title ) {
if ( is_category( 'CATEGORY NAME' ) ) {
$title = 'YOUR CUSTOM TITLE';
}
return $title;
}, 50 );
See more information on WordPress Conditional Tags.