What is the best way to get from Shanghai to Beijing? I will only be Abe to spend 2 days in Beijing.

Taking a direct flight is fastest way for you. It only spends 2.5 hours on the way.
Or you can take high-speed train from Hongqiao Railway Station, get off at Beijing South Railway Station. The duration is 5 to 6 hours. The ticket fare of frst class seat is CNY933 and second class seat is CNY553.
You can choose the way you like.