This does not bode well for when updates come for PB2, PB3 etc.
The direct links for the itunes beta and recovery hd update go well.
But going from DP1 to DP2 via direct links have been problematic. You can download the packages but they refuse to install individually.
So in anticipation (although these instructions are for going from DP1 to DP2, similar instructions may be needed if a similar problem arises in future in going from PB1 to PB2.)
To update DP1 to DP2 via direct links:
1) Download the files FullBundleUpdate.pkg, FirmwareUpdate.pkg and OSXUpd10.12Patch.pkg (links in apple developer forums).
2) Copy the 3 files .pkg.
3) Open /Library/Updates/031-63042 and paste it. If you don't have a folder 031-63042 already here in /Library/Updates, make one yourself.
4) Restart your Mac.
5) Open AppStore and click Update.
6) It should now ask you to reboot and will install the DP2.
7) Reboot and wait to install.
(Note the folder number 031-63042 will be different for each subsequent update. I'll post the correct numbers later when more updates come)