There are about 40 train departing from Shanghai Railway Station to Kunshan everyday. The time schedule could be searched at: https://www.travelchinaguide.com/china-trains/
The Maglev will reach Longyang Lu in downtown Shanghai. At this station, you should transfer to subway Line 2 and get off at the People's Square Station. Then, take the subway Line 1 which could lead you to Shanghai Railway Station directly.

Actually, the airport shuttle bus Line 5 could take you to Shanghai Railway Station directly. By this way, the transporation may be easier~