If you have submitted an application an need to upload or replace a docment first find your application under Applications in your side navigation panel.
Then, find your application and click the grey righthand overview button as seen below:
In your submitted application, find the "Submitted Documents" section. If you did not upload the document it will look similar to the example below. Note it says "No document" and has a blue Upload link.
Click on the Upload link and then select the document you want to upload. You will be presented with an upload window like the one shown below:
After you upload the document it will no longer show "No document" but rather a "Replace" link which can be used to replace the document if necessary. See the example below:
