Robots.txt file not updating

One of our website robots.txt file is not getting updated,deleted .htaccess and robots.txt file from root folder from one of the suggested answers Why does Google Webmaster Tools say my whole site is blocked by robots.txt when I have no robots.txt?

I thought may be some plugin conflict.So I have completely deactivated Yoast SEO and Security plugin which is affecting the file. but still robots.txt file displays me as

User-agent: *
Disallow: /

User-agent: Googlebot-Image
Disallow: /

Any other suggestion or help,which can help me in updating the file will be much appreciated

Thanks in advance!!