First of all, the /u directory isn't the root directory. Secondly, if you're asking how to delete files from the root filesystem I think you need to do a little more research as to why you would want to do this. Then, and only then, use rm <filename> to remove whatever you like. But be prepared - deleting certain files can trash your system and unless it's designed otherwise, rm is very unforgiving, as in there's no recycle bin.