How to respone?

Have a nice day!

"Also, have a good day; have a good one. A cordial goodbye to you. For example, Thanks for the order, have a nice day, or See you next week--have a good day, or The car's ready for you--have a good one. These expressions have become synonymous with a polite farewell. The first originated about 1920 but, like the variants, became widespread only after 1950. "

Have a Nice Day is commonly spoken valediction, typically spoken by retail employees or clerks to customers after a sale, particularly in North America. However in Britain or Australia it is sometimes regarded as an expression of insincerity, its use can annoy people. It is associated with the smiley, a representation of smiling face. It is also the title of several creative works.

How to response?
It depends on. Or you may say: Thank you!

Usually you say You too.

I always say:

Thank you, same to you.
or
Thank you, you too.

多數普遍會有 "Thanks" 及"You too", 或者係兩個一齊用

最common 最簡單就是 You too.

Both are OK.