I have being trying to erase a set of files and folders that I do not longer need but I do not find any option on my phone or on my Ubuntu box. How can I do it?
The Ubuntu One files app on the phone is not a synchronization client. It simply allows you to download some files that you have in Ubuntu One.
You can delete the files from your Ubuntu computer, or on the Ubuntu One web site, and the changes will be synchronized to all the computers you have connected to Ubuntu One. Note that on the computers where you did not perform the delete from, the files will simply be moved to trash. And you will have to manually delete any files you downloaded to your phone which were in Ubuntu One that you don't wish to keep around.