Oh that's not bad. If you had started working on dialing back the heat earlier, they could already outside with those temperatures.
To reduce their heat in preparation for moving out, do some/all of the following:
Continue to raise the heat lamp and/or reduce the wattage of the bulb to drop temperatures, and continue reducing it every 4-5 days.
Turn off/reduce any ambient air temperature around the brooder. Cracking open a window, or not heating the room can help with that.
Take them outside for some play time when it's not rainy or windy. Increase the amount of time they spend out each day so they can get used to the outside temperatures.