If you have root or Reseller access to WHM, you should be able to do this easy enough via Modify Account. I'm assuming by your question that you don't though. If yes, you'll need to speak with your Hosting Provider about this. This is no way for a cPanel user to increase disk space available on their account. That is, other than removing old backups, files not needed, databases no longer used, etc, to free up more space.
