To upload a file into the OnlineGDB editor, look to the left of the green "run" button used to run your program. Select the image of the arrow pointing to the cloud-- this is the "Upload file" button. You can also hover over it for a tooltip to appear.
Reading the numbers from the file into a program varies between each language, so we need to know what language you are using: common languages are python, java, c, c++, etc...