Warming food in the microwave with

レンチンする means to heat food in a microwave. It abbreviates レンジでチンする: レンジ refers to a microwave oven, and チン imitates the bell on older models.

Examples

Heating leftover pancakes with チンする

Manga panel from よつばと! showing example of Heating leftover pancakes with チンする.
よつばと! » Volume 10 » Page 76

During Jumbo’s visit, Yotsuba tells him she made pancakes earlier.

よつば:
「まだちょっとのこってる!チンしてたべるか!?」
“There's some left over! Shall I heat it up to up to eat?”
ジャンボ:
「おう。よつばが作(つく)ったんなら食(た)べなきゃな」
“Sure. If you made them, I have to eat some.”

Lunch is heating with レンチンする

Manga panel from にじいろフォトグラフ showing example of Lunch is heating with レンチンする.
にじいろフォトグラフ » Volume 1 » Page 80

Satsuki is left to watch Sara for the day. As the afternoon arrives, Satsuki discovers their lunch had already been prepared and placed the refrigerator, with a note saying to heat it and eat it.

皐(さつき):
「今(いま) レンチンしてるから待(ま)ってね〜」
“The food's heating up right now, so wait a bit, okay?”
サラ:
「わーい!」
“Yay!”