OwnCloud now supports versions of files on the server. If you have more than one revision of a file you can recover it, so that we can keep a record of the different versions of the file over time. Point your browser to https://owncloud.rp.upf.edu, navigate to the file and click on "..." - "Details".
Finally, it will appear a window with "Versions" option. There, it will list all available versions, clicking on the come back icon, we will recover a old version.
This action will establish the file version in all synchronized devices and it won't be able to turn back.