How to delete my site not found urls?

I have website. last two weeks ago I change new media / entertainment related site. old Content is download link provided. Now I get 20000 urls not found. 5000 urls sitemap errors. How to delete this urls. I am tried. I used google webmaster tools. my s开发者_运维百科ite losing visitors.

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You should redirect the old locations to the new ones, using the HTTP 301 Moved Permanently status code.

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Redirect old url to the new one! This can be done in .htaccess fle:

Redirect 301 http://old_url http://new_url