take this as a example,the current xml only show the newest 42 news
but i want to access to older feed, how can i do that?
There are no parameters to do that. Some feeds like the weather do take a couple of parameters, for example what location are you interested in, but you are stuck with what Yahoo returns to you. Let Yahoo know what you need. Maybe in the future they will evolve their developer access.
If you want to read more about the evolution of Yahoo's offerings, you can start here. The news search API, which has been deprecated for 1.5 years did allow for a start and results parameter to specify how much information you got back.
I also urge you to examine YQL. You can play around with different "queries" here to see how you can customize your RSS retrieval experience. Note that the YQL query will yield the same number of results. If I try to change the number of results with my YQL query like so, I still only get back 40 hits so the news rss is limiting what I can get back.