How can I edit the robots.txt file?

I am trying to modify my http://www.opc-1-2-3.com/robots.txt file and I do not know how. It is not a StoreFront as shown above. It is just a content website. It is supposed to be located in the root directory of the file manager, but it is not there. The reason to ask for this is that a Chinese search engine (baidu.com) says that this site is not accessible because it is blocked by the robots.txt file, while there is not such a problem with all other search engines. It is strange. Thanks.

Note: This topic was created from a reply on the Can I modify the robots.txt file? topic.