You can take the ATB Bus n. 1 direction to FFSS STAZIONE/CITTA’ ALTA and take off in Bergamo’s city centre at the railways station or directly at Upper old town (Città Alta).
This bus runs every 20 minutes from 5 am to 00 am and it costs €2,30 oneway.
You can take the direct bus Orio Shuttle (Link) from Terminal 1 and Terminal 2 and it will bring you directly to Bergamo-Orio al Serio Airport and then you can follow the same directions as wrote above “Milan-Bergamo Orio al Serio Airport”.
Or you can take off at the last stop in Bergamo’s city centre, VIA FURIETTI n. 17.
You can take the direct bus to Milan city centre, it costs ca. €9 oneway and buses run every day from 5:30 am to 10:00 pm.
From Milan city you can take direct bus or train to Bergamo as written here below.
You can take the direct train that brings you to Bergamo in 60 minutes, it costs €4,80/5,50 oneway (€9,60/11 return).
The train from Milan Porta Garibaldi to Bergamo and vice versa runs 2 or 3 times per hour from 5 am to 10 pm.
There are domestic direct flights to Bergamo only from Alghero, Bari, Brindisi, Cagliari, Catania, Crotone, Lamezia, Palermo, Pescara, Trapani; these flights are operated by Ryanair (Link)