If you have made a few errors with your robots.txt file by letting Googlebot crawl something you simply did not want it to or if you accidentally blocked Googlebot, you have to wait till Googlebot picks up the new robots.txt changes.
Google has introduced an addition to their robots.txt tool. With the help of the changes, you can not only do the usual check of your robots.txt file, but you can also verify the live version and submit it to notify Google about the change in your robots.txt file as well as request Googlebot to crawl it again.
To access the new tool, you need to select “Crawl” within Google Webmaster Tools in the left sidebar. You then need to choose the robots.txt Tester.
You can find the change to be live in Google Webmaster Tools accounts.Robots.txt Update Tool Added to Google Webmaster Tools!,