heat up the food

can help me translate into japanese. when i want to buy a bento that is cold. How to ask them that I want them to help me heat up the food?

"Atatamete kudasai." (Please warm it up).

"atatakai" = (adjective) "warm"
"atatameru/atatamemasu" = (verb) to warm up, to heat up (something)
"atatamete" = (the "te-form" of the above verb)
...combined with "kudasai" makes it into a request.

If you buy at a convenience store, they will normally ask you "Atatamemasu ka?" (Should I heat/warm it up?)

By the way, the microwave oven is called "renji"(range) in Japan.
If the staff thinks you can't understand "obento atatamemasu ka?", maybe he ask you "renji" or something.
Because many Japanese people think "renji" is correct English. (I had thought so too.)

As AK said, "atatamete kudasai" is correct Japanese. But I think also Japanese people can understand your request by saying "renji" or "hotto"(hot).

As someone who used to teach the Japanese language to non-native speakers, I believe in teaching Japanese "as it is" spoken in Japan, not artificially simplified for the sake of non-natives, unless it over-complicates things :) Thus such was my answer.