1. 应该填入which.
它是关系代词,在该定语从句中它代替先行词Guangzhou,并充当介词from的宾语.两者又一起充当该定语从句中的地点状语.
from which we went to Shanghai
=from Guangzhou we went to Shanghai
=we went to Shanghai from Guangzhou
2. 翻译：我们在广州待了两天,从那里出发我们又去了上海.
3. 如果该定语从句没有from这个词,可以单独填入关系副词where来引导这个非限制性定语从句.
关系副词where=介词from + 关系代词which