If there is no beta 6 for iOS 9, Apple has still to release the beta of OS X 8 El Capitan for developers, in addition to the sixth beta public version.
If you are already on the previous beta (for devs) then you can do the update from the Mac App Store, otherwise grab a copy on the Dev Center. For other Public Beta, you only have to go again on the Mac App Store to download the new version.
The El Capitan OS X version was probably aware happen this fall. Apple certainly bring information on the iOS 9 final version during the keynote, if nothing is published by then. We’ll be sure to let you know if the iOS 9 beta 6 appears tomorrow or Wednesday.