Just a reminder (as I sit melting on a London bus): if you've been finding yourself needing a sip of water or another pint glass of something-or-other in the heat over the last few days, you're baby will be needing more hydration too! 

If you don't normally feed on demand and don't anticipate feeds until scheduled times, your baby will need some flexibility here with more regular feeds offered! 

If you breastfeed, just offer the breast or breastmilk via bottle. Your breastmilk adapts to your baby's needs so, in the heat, it will be very hydrating for your little one. However, just like you and I, your baby is also likely to be more thirsty (and just generally irritable in the heat) and is likely to therefore be more demanding, more frequently, for the boob! Keep feeding on demand, reading your little one's cues, and your little one will hopefully be content (or as content as anyone can be in this weather...can you tell I'm a cold weather person?) 

If you formula feed, you could offer more regular smaller feeds or cooled boiled water (in small amounts) between feeds! For a baby over 6 months, you don't have to boil the water first. Since every formula feed remains the same i.e. the water content in your chosen formula doesn't magically increase when the temperature outside does, be wary of overfeeding your little one by offering full bottles in response to thirst as well as hunger. 

Excess water for young babies under 6 months can be dangerous as their kidneys are still developing. It also might risk filling up your baby's tummy leading to missed milk feeds. Since milk is where your baby gets his nutrient requirements, it's important not to replace this with water.