How do I do 'so many crawling errors in Webmaster Tools' in SEO?

Steps to fix  'so many crawling errors in Webmaster Tools' in SEO:

• Errors in Server Connectivity-The search engine crawler has been completely unable to access your website if Google Webmaster Tools reports a server connectivity issue. Such issues are typically caused by a website's sluggish response time or a setup error that prevents access .If server connectivity issues only happen once in a while, it's most likely due to your hosting provider being momentarily unavailable due to maintenance or a technical issue.
• Errors in DNS DNS problems, like server connectivity errors, occur when Google is unable to contact with the DNS server or locate your content on the server. To ensure access, you'll need to contact whoever manages your DNS records and web hosting server once more.
• Issues with Robots.txt-Robots.txt is a tiny text file located in your Web server's root directory that lists any URLs on your site that you do not want search engine crawlers to access. There are a variety of reasons why you might not want specific URLs indexed, and any pages that are limited by the robots.txt file will be referenced in Google Webmaster Tools.
• This page was not found (404 Errors)-When a link leads to a webpage that cannot be located, the iconic 404 error displays. While a 404 URL error is intended to indicate that the page being linked to does not exist, the issue is frequently caused by a broken link or another technical issue. You'll need to redirect any offending links to an existing and relevant page on your site if you're getting a 404 error.
• 404 Not Found-Errors with the code 'Soft 404' are also possible. When a page cannot be found and is redirected, Google believes the content on the destination page is not relevant or valuable enough.You might want to redirect these pages to a more appropriate URL on your site.

Read More: How do I remove crawl errors?