If you mean to delete compilation database files and any other files generated by compilation, just use Project menu -> Clean Project.
Thank you for the response.
There is no option as clean project in the Quartus II software. Here is the screenshot of the project menu options. Can you help me to find out?
Seems like you are using the old version of Quartus. This feature is not implemented there yet. Can you use the latest release of Quartus Prime?
Quit Quartus and delete the db and incremental_db folders. It does basically the same thing.
For more complete information about compiler optimizations, see our Optimization Notice.