legal ground
Article 51 of the Detailed Rules for the Implementation of the Patent Law of People's Republic of China (PRC).
The applicant for a patent for invention may, within 3 months from the date of receiving the notice that the application for a patent for invention has entered the substantive examination stage issued by the patent administration department of the State Council, take the initiative to propose amendments to the application for a patent for invention.
An applicant for a patent for utility model or design may, within 2 months from the date of application, put forward amendments to the application for a patent for utility model or design.
Article 33 of the patent law
The applicant may modify his patent application documents, but the modification of the application documents for patents for inventions and utility models shall not exceed the scope recorded in the original specifications and claims, and the modification of the application documents for patents for designs shall not exceed the scope shown in the original pictures or photographs.