Mysql: Update field of most latest record

You could try using ORDER and LIMIT.

Try this:

UPDATE messages_tbl SET is_unread = 1
WHERE name = 'John'
ORDER BY ReceiveTime DESC
LIMIT 1

This query will update the rows in order of the highest (most recent) ReceiveTime to the lowest (oldest) ReceiveTime. Used in conjunction with LIMIT, only the most recent ReceiveTime will be altered.

You can join both and perform update based on the condition.

UPDATE  messages a
        INNER JOIN
        (
            SELECT  name , MAX(ReceiveTime) max_time
            FROM    messages 
            GROUP   BY name 
        ) b ON  a.name = b.name AND
                a.ReceiveTime = b.max_time
SET     a.is_unread = 1
-- WHERE    a.name = 'John'

Without the WHERE condition. It will all update the column is_unread for the latest entry.