能加热一下吗？

Just point at your item and say the phrase. You can add 请问 (may I ask) at the start. This makes it even more polite. In a convenience store, just hand over your bento. The staff will know exactly what to do. You don't need complex grammar here. Just the phrase and a nod will work.

You will use this most at 7-Eleven or FamilyMart. It is perfect for those refrigerated steamed buns. Use it at home with roommates too. If your soup arrives lukewarm at a casual diner, say it. It is also great for office life. Use it when a colleague is near the microwave. It is a very practical, everyday tool.

Don't use this at a high-end Michelin restaurant. It might imply the chef failed their job. Avoid using it for things that shouldn't be hot. Don't ask to heat up your salad or sushi! Also, don't use it for people. Heating someone up has a very different, awkward meaning. Stick to food and drinks to stay safe.

Chinese culture places huge importance on hot food. Cold meals are often seen as bad for digestion. This is why you see hot water dispensers everywhere. Even in summer, many people prefer warm tea over ice water. Asking to heat your food is culturally expected. It shows you care about your health and comfort. It is a very "pro" move for any visitor.

If you are in a rush, say 热一下 (heat it briefly). If you want it very hot, say 热一点 (a bit hotter). You can also use 帮我热一下 (help me heat it). This adds a nice touch of gratitude. In some regions, people might just say 加热吗? as a question. But the full phrase is your safest bet for being understood.