1.0 Cache Clearing
First, go to your user settings page and join the beta program. https://circleci.com/account/beta
This will enable the "Clear Caches" menu option on your project settings page. You can navigate there by adding /edit#clear-caches to the end of your project page URL:
2.0 Cache Clearing
With 2.0, the only way to clear your project cache is through the cache keys you create in your config.yml. You can add a versioning prefix to the beginning of your keys and increment them when you want to clear out and rebuild the cache. You can read more about this process here in the 2.0 docs:
Still having cache issues?
If you've cleared your project's cache and still feel like you're having cache related issues, submit a support ticket and we'll take a closer look.