In the Filings section, expand the details for the filing where you would like to replace the document. To do this, click the pencil icon on the right for the line item. 
Scroll down to where your document is uploaded. To remove the document, you can click the trash icon under Actions. This will remove the document and provide you the opportunity to upload the intended document.
To upload, click "Browse" to select your PDF document or drag the PDF Document to where it says "Drag file here"