Google has some 2 year old outdated meta tag description as my SERPS description on competitive keyword. How can I change it?
Change your meta tags description, unless you mean you have and it still has your old description then I would agree with vansterdam and upload a xml sitemap, but also you should upload a regular html site map too.
If you've updated your meta description tag but not the content then it will take longer to get updated than usual. However at the ends it will get updated and all you need is to got the bot to visit your website and cache the new pages
If you have updated them on your site, Google will eventually get round to updating them, it just takes time. You could try digging your site so Google will visit it sooner.
Make your description more relates to the keywords, such as put in the front of the description, repeat twice etc, google might pick up the descrition and updat it in their datacenter.