Hello I applied for broadband a while ago and it starts next Tuesday, but now I found black Friday deal, so I want to change it
You would have to cancel your existing contract and then apply for a new one, once its been cancelled. However the deal you want may not be available then, and you may also have to wait a long time for a connection, and may even lose your allocated port in the cabinet, and it may be full, especially if lots of people apply for the new deals.
Promotions are offered all of the time.
If you bought something in a shop, and then saw the same thing a week later at a lower price, you would be lucky if a retailer would take the item back, especially as you would have used it.
If your ordered package has not started yet and presumably you are still in the "cooling off" period, you should contact BT 0800.800.150 and tell them you wish to cancel the order.
Once it has been cancelled you can then order the cheaper deal, however, as has been pointed out you may find that there could be a delay in getting the package up and running and all most certainly not by next Tuesday.