Something

TABLES:
tbl_data (id, newtime, changetime, ticket_info)
tbl_change (ticket, comment, time)

SELECT DISTINCT `ticket` , MAX( `time` )
FROM `tbl_change`
GROUP BY `tbl_change`.`ticket`;

UPDATE `tbl_data `
SET `tbl_data`.`changetime` = ( SELECT DISTINCT `ticket` , MAX( `time` ) FROM `tbl_change` GROUP BY  `tbl_change`.`ticket`)