I have found out that one of my pages has weak SEO and shows in the later pages of google. So I modified my page a little and added some description etc. Now is there a way to tell google that I have updated this single page? I could not find it in webmaster tools? Did I miss it ?
Few of things you can do:
1 - On Webmaster Tools, Go to "Health" / "Fetch as Google" where you can enter the address of the page that has changed, click on "Fetch" then click on "Submit to index". At which point a message will pop up and say: "Select if your page is new or has been recently updated. Google doesn't guarantee to index all submitted URLs." Choose your option, and hit OK.
2 - Resubmit sitemap.
3 - Adjust crawl frequency.
Googlebot will recrawl soon, so there is generally nothing you need to do. How soon depends on the pagerank of the page. Here are some rough guidelines about how long it might take googlebot to return.
You might be able to get googlebot to come back faster by bringing the page to its attention. I know that for new pages, you can tweet about the page on Twitter. Then googlebot will crawl it right away. I haven't tried getting Googlebot to return to an updated page with a tweet, but that might work as well.
In Webmaster tool, add the updated link in your existing sitemap.xml and re-submit it.
Otherwise, If your site is a blog, you can use pingomatic.com. So it will ping your site to the major blog search engines.
I hope this will help you.