Come to the title of the paper in your Contributions and see written below it "Upload full text". Then follow the instructions. PDF of the papers are desirable.
As you said, you have to scan your papers to covert them into e form. Then, on your own web page, researchgate provides you with clear instructions. In fact, after clicking on Manual entry, you will be guided to feed in every paper's general information, and afterwards, you click on Select file for uploading the full text.