Depends what kind of cache. You can go into settings, apps, manage apps and do it the long way app by app. Or go into your browser and do it from the browser settings bc the browser seems to build the most cache. Or go into liberty settings and do it there. and or if you want to wipe the dalvick cache reboot into recovery, advanced, wipe dalvick.
Either way your e freeing up space and should notice a slight speed increase
Edit, I just noticed you said when, not how to. Duh