Before anything, you should create a child theme, so you can update the parent (Twenty Fifteen) without worries.
Looking at the WordPress hierarchy, you could copy the archive.php to the child theme and edit it, but would need to check what archive it is, of posts, custom post types, etc.
Instead, copy archive.php twice, rename one to category.php and one to tag.php. You could even use category-{slug}.php for specific categories (same goes for tags).
// in category.php / tag.php / ...
the_archive_title( '<h1 class="page-title">', '</h1>' );
_e('Some custom text','twentyfifteen-child');
the_archive_description( '<div class="taxonomy-description">', '</div>' );