After adding a document folder to a transaction, from your right side panel, click the Upload button to have your file stored in the system.

From the Document Folder, click the + File button then enter the file name, then click Add. Click the Upload button to have your file stored in the system. The Placeholder File (+ File) is a place where you can put your files inside your document folder.

Once the document is uploaded to your folder, you can change the file by clicking the Replace button. The original document will not be stored anymore and it will only keep your replacement document.