How to postpone M5 Mac Studio delivery and manage trade-in?

He means that you contact the online store by telephone, provide them the order number and then tell them you want the delivery done to your local Apple Store instead of your home.

You can find the online store's phone number on their main page. If you happen to be in the USA, you can click Contact - How to Contact Us - Apple

to find the phone number. If you are outside the USA then use Google to find the on-line store's phone number for your country.

 Apple does not allow you to change or postpone a delivery date once an online order is placed, but you can use an Apple Store pickup strategy or trade-in extensions to buy yourself more time. 

What date are you given if you go through the motions of ordering another one with the same specs now? (You can abandon your new order before completing it, and you still can see the proposed delivery date.)

if it gives you are more convent date (albeit later) it may be better in this case to cancel you original and take a replacement.