To be frank, I think your plan is a little urgent since you only have 6 hours freely. Usually, the bus takes about one hour to reach the urban area from the airport for a single way. But, consider this one. Firstly, take the Maglev from the airport to its terminal-the Longyanglu Station, then transfer to the Subway Line 2, by this line, you could reach the Nanjing West Road of Shanghai. It is the most prosperous street in Shanghai. You also could have a visit to the famous Bund at the end of this road. Finally, return the airport by the same way~
Good luck！