This example will modify the search value of a query in an entry search.

add_filter( 'frm_filter_admin_entries', 'custom_query', 10, 2 );
function custom_query( $query, $args ) {
    $fid = 'id';
    if ( $args['field_id'] === $fid ) {
        $query['it.' . $fid][0] = 'new value to search';
    } 
return $query;
}

Use with frm_admin_search_options hook