If you've received an e-mail telling you that your paper has been accepted and asking you to submit a full paper before May 1, here's how to proceed.
Log in to sciencesconf.
Click on “My submissions” (left - A) and click on the “pen” icon (= edit - B) opposite the title of your paper (in the central part of the web page).
This takes you back to the first page of the proposal submission process. There's no need to change anything on this page. Click on “Next step”.
This takes you to step 2 (authors), and again, there's nothing to change. Click on “Next step”.
This takes you to the file deposit page. In the “Additional data” section, upload your complete article. Click on “Choose file” (A), select your article on your computer, then click on “Upload” (B).
The document you've just uploaded appears at the bottom of the page (A). You can edit its properties if you wish (by changing the file type, for example), but this is neither necessary nor useful. Click on “Next step” (B)
You've reached the end of the process. Click on “Submit”