Hade problem med senaste betan för El Capitan (nr 6). Fastnade på 111 MB och sen kom den aldrig vidare. Hittade den här lösningen som jag testade och nu fungerar det.

Based on dfooter’s experience below, it seems that App Store gets struck after downloading the firmware part of the update. So the first thing to try is to go to /Library/Updates/ and run the 107.6MB FirmwareUpdate.pkg file you find in there. That will install the firmware update and free up App Store to download the Beta 5 update.

via Update with several El Capitan Beta stuck at 10… | Apple Developer Forums.