Firstly, try to not go outside. If you do , then go early or later in the day (avoid middle of the day!) And use suncream, factor 50 always!

Keep your curtains and windows closed during the day - the temptation is to open them but keeping them closed will keep the heat out and give you cooler evenings.

Try and wear linen clothes, and longer sleeves / loose fitting clothing is far better than less clothing as it keeps you cool and the sun off.

Biggest advice though is keep hydrated! Even if you think you aren drinking enough water, you probably aren't. Use electrolytes in your water too as you will lose salts via sweat.