How to overwrite shared document

Hi

I've been granted : 'Read,Write' on a document. I downloaded the document that was shared to me, made changes and tried to upload it back. But the only place I can upload back is under my own folders. I thought that since I am granted 'Read, Write' on that shared document, therefore I should be able to overwrite the shared document? Or am I missing something? I am trying to overwrite the document that was shared to me so others who have also been shared can see my changes to the shared document.

Thanks
Angela