how can I upgrade from 3.0.4 can I just overwrite the files with the latest version? Thanks,

Short answer, yes. AFTER backing up everything - including your database. However you will need to run the database upgrade scripts in each version in sequence. But you can overwrite the 3.0.4 files with the 3.0.15 files.

Personally, I'd backup my database and install 3.0.15 fresh into a new directory with a new database and then import the backed up database into it. This preserves the 3.0.4 store until you are sure 3.0.15 is ready to run the way you want it so minimizes the downtown for the store.

Thanks done it and it went quite good, there was nothing worth doing backup and everything came out clean, thanks!!

No problem:)