how can i extend my stay in china?

Usually, you can apply for an extension of your S2 type 7 days earlier before it expires at the exit and entry administration of the local PSB, but currently, stay duration of your S2 type is 7 days, I’m not sure if you can apply for an extension.
Or you can try to change S2 type to S1 type, but this should also be done 7 days earlier before S2 type expires. You can prepare all the required materials ahead of time, and after entering PRC, go to the exit and entry administration to convert S2 type to S1 type.