Are you using a Native Installer, a Virtual Machine or a Native Installer? Depending on the platform, the way to access the code of Osclass is different. For example, on a Cloud Image you need to log into your server via SSH and then you can use a command-line editor to modify any file you desire.
On the other hand, if you want to access the database you can use phpMyAdmin. You just have to follow the steps described in the guide below:
I hope this information is useful for you. If you have any other question, please do not hesitate to let us know.