When you want to work on the document, you first have to download an editable file, then do the changes you want and upload it again.
In order to download the file, you must click on the Import/Export Icon, then select Export. You can choose the format of the file and the name of the file which will be created. By default the name is 'Reference-Version-Language'. It is a good name since it create a unique file name for this document.
Once you download the file, you can edit it with the proper software. We recommend you to use Open Office.
Once it is finished, you can re-upload the file as you did before. Click on 'Browse' on the main tab of the document, select the document on your computer, then click on the 'Save' button.
You can see that the revision number has increased. Each change is recorded and kept in the system so you don't lose any information.
Note: Be careful when you have the document on your computer, anyone can download, edit and upload the new version of the document. If you upload your file after, you will erase his/her changes. You must perform your changes as quickly as you can, and make sure that no one else is working on it in the same time.