They were in Shanghai last Sunday.
改为复数句,就是把主语变成复数.“人或物体”的复数都是they,所以主语就改为they了.动词也要改变,修饰复数的动词为are,又因为是last Sunday,过去发生的事情,所以用were.
希望可以解决您的问题,不明白的随时追问,