From the xCenta Dashboard, click on the Documents icon in the Modules section.
A new window will appear. Click on the folder on the left side and it will bring up all the sub folders contained within. Click on the folder that contains the document you wish to edit. Click the 
icon on the document you wish to edit.
A new pop up will appear. Edit the document, then click OK.
If you are unable to follow these instructions, you may not have the required user permissions. Please contact your Site Administrator.