i see people asking about visa extension all the time. thought i would share my experience because it is actually not that hard.
i had a 30 day tourist visa but wanted to stay 15 more days. went to the local exit entry bureau in shanghai. not the airport. the city one.
bring your passport, a passport photo, your hotel registration (the little paper from the hotel), and money. i think it was like 150 yuan.
the staff spoke very little english but i used google translate and pointed at things. they were helpful actually.
took about 2 hours total including waiting. they gave me a 15 day extension no questions asked. but i heard it depends on the city. beijing is stricter they say.
one important thing. apply before your visa expires. like at least 3 days before. if you wait until the last day they might say no.
also not every city does extensions. smaller cities might tell you to go to a bigger city. so plan ahead.
if you are on a visa free entry (like 144 hour transit) you cannot extend. you have to leave and come back.