To Heat Something Up
To Warm Something Up
transitive verb, ichidan verb
You know that the kanji 温 means warm, and this verb is related. It means to heat something up or to warm something up. It's all about you doing something to another object to make it hot, like putting it in the microwave.
The kanji reading is the same as 温かい. As long as you know that reading, you should be able to associate it with this word.
Just like you can say あったかい for 温かい, people often say あっためる in conversation. The "official" reading is still あたためる, so when typing out the kanji or taking a test, try to stick to あたためる.
おにぎり、温める？
Do you want to warm up the onigiri?
牛乳を温めてホットミルクにして飲むのが好きです。
I like to heat up milk and drink it.
そのアイディアは私がずっと温めてきたものです。
That idea is something I've been nurturing for a long time.