If you are the author of the entry or have admin permissions you will be able to Edit a community asset.
It could either be a created discussion/document or an uploaded file/video that you´d like to replace with a new one. For any of those cases, editing is pretty easy, go to the community entry you´d like to update and click "Edit" from the right side Actions menu:
You will be able to upload a new file (if that´s the case), and/or edit the title and description of the Community asset:
You could also update its Categories or Advanced options (restrict authors, require approval before publication, restrict comments).
You can choose to notify all people following the space where document is being stored, people following the document or participants involved in any discussion (comments) based on such entry, by leaving the check-box empty for "Minor edit, don´t send notifications or update streams".
If you wish to do the opposite check the box, so that no one if notified about this change and then Publish your editions.