You can find it under the developer menu:
After pressing it, the cache will be cleared.
Just make sure to be in an Empty domain, such as Empty, as otherwise it will get refilled with all the assets in the domain (so avoid places such as hq/playa or even your own sandbox).
Additionally, if using windows you can do this manually by going to
and then deleting the data8 folder. That contains the interface asset cache. Cache folder it self contains the web-browser cache. The Reload content button in the Developer Menu basically does the same thing, but if you do it manually, you have to reset the client.
Just note that doing this will cause you to download absolutely everything again.