There's a regular shuttle bus service that operates between the central train station and Linate from 6:30 am but that clearly is too late for you. I think ATM's Bus 73 from Milan Duomo to Linate operates 24/7, you can find the times and route on Google Maps and plan accordingly. If you're not staying close to a Bus 73 stop your best option is probably a cab, which should set you back about €50.

https://moovitapp.com/index/en/publi...-6927-586839-0