Fetching details from the dictionary...

to cool (water) down; to make less hot (e.g. by adding cold water)
「温める」 has other readings with different meanings.
沸かした熱いお茶をぬるめた。
I cooled down the hot tea.
熱い味噌汁に水を足して温めて飲んだ。
I added water to the hot miso soup to cool it down and drank it.
赤ちゃんのミルクを水で温めている。
I'm cooling down the baby's milk with water.