[Gary MichaelSupport Manager]

Try this.

add_filter( 'syncs3_merge_tag_presigned_link_time', 'my_syncs3_merge_tag_presigned_link_time', 10, 5 );
function my_syncs3_merge_tag_presigned_link_time( $time, $url, $entry, $field_id, $form ) {
	return '+60 days';
}

You need to change the +60 days accordingly.