Babies don’t like their milk cold and they sure can’t take anything piping hot. Whether it’s stored breast milk or baby formula, you still have to warm their bottles.
If you’re giving the baby frozen breast milk (to best preserve it), you have to thaw it thoroughly. When thawing, leave it on one of the fridge racks to thaw overnight. Leaving it in room temperature will make it prone to spoilage or attract germs. For a quick fix, you can place the bottle under running tap water.
To warm the milk, you can submerge the bottle in a bowl of warm (not boiling!) water. Commercial baby bottle warmers are also available. Test a few drops onto your inner wrist. It should feel about body temperature (just like breast milk) and not hot at all.
Do not use a microwave. The heat can destroy some of the nutrients in the milk or formula and microwaves can heat it unevenly and cause hot spots which can scald your baby.
Some babies don’t mind taking milk in room temperature. If you’re using baby formula, try making it using bottled water at room temperature. “Heat” the bottle by clasping it in your hands for a minute or two. Try it out with your baby. There seems to be no medical consensus that having the milk or formula at body temperature is better for the baby health-wise.