I have extended a tourist visa in China before in Guangzhou, which is a big city with a lot of foreign expats. The building was dedicated to this stuff and the process was exceptionally smooth. It was easy and efficient even though I only had about one day left on my current visa.
Without VPN I have no Google and other search engines are absolutely useless for finding this info about Xi'an, even though it's also a major city. (Please don't answer or comment telling me to get VPN. Ask me in chat if you want.)
Other answers I find for other cities or smaller towns seem to be much more involved than what I had to do in Guangzhou. If it's that complicated here I might not bother. If it's easy like in GZ I'll probably do it. For instance many say you need a week or more left on your visa. This was not the case in Guangzhou at all.
Is there an official site on this for Xi'an? Or can somebody find a traveller's description of the process in Xi'an? (Blogspot is blocked in China so if you find somebody's story there, please add the key points in your answer.)