Breast milk does not need to be warmed up or heated. It can be served at room temperature without any issue. However, if you still want to heat it up a bit, you should never boil, heat, or microwave breast milk directly. The best way to do so would be to submerge the container with breast milk in warm water for a few minutes. This will heat up the content of the container. However, once you're done, check the temperature by putting some on your wrist. The milk should always feel warm, not hot.