In case you are referring to the title shown in the browser title bar: this could be achieved by coding, but the easiest way would probably be to use an option provided by the SEO plugin Yoast (in case you have that installed).
In your admin go to “SEO > Titles & Metas > Post Types” – then at the end of that settings page you can choose a pattern for the Archive pages.
PS: In case you are instead referring to the actual page title, you can change or actually override that by copying the archives template archive.php to your child theme and then editing that.
If you find “Archives” added to your page title in the template then most probably the template adds that to the category term. In Twenty Twelve the respective code looks like this:
<h1 class="archive-title"><?php
if ( is_day() ) :
printf( __( 'Daily Archives: %s', 'twentytwelve' ), '<span>' . get_the_date() . '</span>' );
elseif ( is_month() ) :
printf( __( 'Monthly Archives: %s', 'twentytwelve' ), '<span>' . get_the_date( _x( 'F Y', 'monthly archives date format', 'twentytwelve' ) ) . '</span>' );
elseif ( is_year() ) :
printf( __( 'Yearly Archives: %s', 'twentytwelve' ), '<span>' . get_the_date( _x( 'Y', 'yearly archives date format', 'twentytwelve' ) ) . '</span>' );
else :
_e( 'Archives', 'twentytwelve' );
endif;
?></h1>
…but in your theme this will probably look different, hence the title structure you described. It’s difficult to say without knowing what theme you use and/or what your templates look like. I hope this helped anyways to digg in the right direction.