"Heat it up again" would be common.
Student or Learner
The milk for the baby is not warm anymore and have to warm the baby's bottle.
Do you say "It's a little cold, I have to heat it again.” or "It's a little cold, I have to warm it again.”
"Heat" or "Warm" is the more correctly used in the sentences above?
For me, you could use:
- Heat it again
- Heat it up again
- Heat it through again
- Warm it up again
- Warm it through again
I believe you don't give "hot" milk to babies, just milk which has been warmed so I would be more inclined to use "warm" in the statement but I'm quite certain you will hear people using "heat".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
You can also say 'reheat it'.