If you have any doubts about your situation you should enquire at a Chinese Consulate or at a Public Security Bureau Entry and Exit Administration office within mainland PRC. Any postal or couriered applications from individuals are not accepted. 

Application in China (Renewals and Extensions) 
It is not possible to renew, extend a visa or change into another type. However, if you want to spend more time here, want a multiple entry, or need to have a different type, you can - in many circumstances – apply within China. The procedure is very similar to applying outside through Chinese embassy, but you must go to the Public Security Bureau (PSB) Exit and Entry Administration office for the municipality where you will be 'resident' on the night following the submission of the application. Smaller centers might not have good English speakers so it is a good idea to take a Chinese friend with you if you can. Ask at international hotels if you are not sure where the PSB Exit and Entry Administration office is. 

The new visa runs from the date of issue (a small difference compared with those issued outside China which do not start until entry) and the old one is cancelled at that time. So, you cannot have two concurrent visas such as a student X and a work Z. It is not a good idea to apply until the old one is about to expire as you want to use every day issued on the old one. According to the new regulation 2013, you have to submit application for extension at least 7 days before its expiration. The PSB officers will issue you with a return receipt once they accepted your application and make the final decision within 7 days. That is to say, there is no gap between visas. Because when you get the new one, the old one becomes invalid. If you are late applying, you have overstayed and are subject to a fine and possible expulsion. 

If you run into difficulties when at the PSB, it is a waste of time getting irate or trying to bluster your way through, it simply doesn't work. Stay cool and you will generally find the officials help and explain all they can. 

Currently, it seems that one month is the standard period for regular tourist L extension issued in PRC. Longer ones are available in some situations such as for persons married to Chinese nationals. 

Complete the application form from the PSB Exit and Entry Administration office for the locality where you are registered as residing the night of the application, (ie AFTER you apply). It is probably easiest to take the application in yourself. If you have a friend submit your application there are additional regulations.

The section what you must submit for application explains what you need to get together before you apply. One point is that you must include 'photocopies of your passport bio-data page, your current visa, and your last entry stamp; financial support with RMB 21,000 for a 30 days extension (USD 100 per day) - a stamped balance inquiry from Bank of China will do the trick; and a residency registration'. The residency registration comes from the regulation that all foreigners must register their accommodation details within 24 hours of arriving (48 hours in remote areas) – as explained below. 

If you are staying in a hotel certified to accommodate foreigners - this excludes most budget hotels, but youth hostels affiliated to the Chinese YHA are ok – they can provide you with a hotel accommodation registration slip, (旅馆住宿登记 lü guan zhu su deng ji) for the night of the date you are making the application, not the one before! If you are not staying in a hotel, you must obtain an accommodation registration printout from the police station for the area you are living - and where you should already have registered. So if you are staying in a non accredited hotel or with friends or in an apartment but have not registered, you should either register or stay in an accredited hotel for one night in order to get the slip.