start by creating a child theme for the edits.
then, for example, create a folder /template-parts/post/ in the child theme and add a file content.php into it, based on a copy of content.php in the corresponding folder of Twenty Seventeen.
then edit this section:
/* translators: %s: Name of current post */
the_content( sprintf(
__( 'Continue reading<span class="screen-reader-text"> "%s"</span>', 'twentyseventeen' ),
get_the_title()
) );
to:
if( is_category() ) {
the_excerpt();
} else {
/* translators: %s: Name of current post */
the_content( sprintf(
__( 'Continue reading<span class="screen-reader-text"> "%s"</span>', 'twentyseventeen' ),
get_the_title()
) );
}