To warm (up) or heat something (transitive): to raise the temperature of an object, food, beverage, or body. Often read あたためる in everyday use.
お弁当を電子レンジで温める。
I warm up my boxed lunch in the microwave.
寒かったので手をこすって体を温めた。
Because I was cold, I rubbed my hands to warm my body.
ミルクを人肌くらいに温めてから赤ちゃんに飲ませる。
Warm the milk to about body temperature before feeding the baby.
その親切な言葉が彼の心を温めた。
Those kind words warmed his heart.