You will only be able to upload/ change any documents when you are completing the envelope, if you have already completed the envelope and need to make any changes to the document's you have uploaded you would need to contact the production you are currently working on. They will then re-send the envelope to you so you can make any adjustments. 

When completing the envelope that has been issued to you navigate under the 'Form Details' section.

To upload a new document, select the yellow attachment icon, navigate to where you have the file saved and this will then upload the document to the envelope. 

If you have already uploaded a document you will see a bin icon located on the right hand side, select this to remove the uploaded document like shown below.