Add this on a child-theme’s functions.php
Replace the number 10 with your chosen category ID.
function frontier_home_category( $query ) {
if ( $query->is_home() && $query->is_main_query() ) {
$query->set( 'cat', '10' );
}
}
add_action( 'pre_get_posts', 'frontier_home_category' );
Get the category ID by going to the category page ( Posts -> Categories
). Click on the category you want then on your browser’s address bar you can search for the number ( ex. tag_ID=90 )