Are the files on a plug-in media like a USB drive ?
There are reports of drives being mounted as read only.
If this is the case, then more details needed.
If the files are 'your' files, then you should have read write access, so again more details needed.
If the files aren't user files, then a typical user won't have write permission by default.
File permissions in linux are there for solid reasons, but there are many good articles if you want to read up.
Search for something like "ubuntu file permissions explained"