Your question seems vague but I'll try to answer as best as I can. I'm assuming you want the last id in the table. MAX() is a summary function and isn't really what you want to use to get the last id in the table. It will work but the results can be pretty sketchy.

UPDATE <table> set item_name='bar' WHERE id IN( SELECT MAX(id) FROM <table>)

could be rewriten as

UPDATE <table> set item_name='bar' WHERE id = LAST_INSERT_ID(table_name.id)

I'm not sure why you would want to do this as record updates should always be updated against a primary key. You're asking for trouble if you don't. Both statements above will always update the last inserted record in the table and have a high possibility of messing up the records.

Hope this helps and John please step in if I'm wrong.

The Newt