I am trying out the Windows 10 Technical Preview, and have been running build 9926 for some time. Today (19032015) Microsoft released build 10041 and I installed it immediately, of course. Not surprisingly I had some problems which were so bad that I reverted back to the 9926 build. I later figured out that it might not have been the new build that was the problem, but something else. So I wanted to try installing 10041 again to test that theory. Problem was that 10041 was no longer being offered to me in Windows Update. Turns out Windows keeps track of the builds you have reverted from and hides those from Windows Update. Here is how to make them visible again.
In Registry Editor, navigate to HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsSelfHostApplicability.
This is where all the settings for the preview program are stored. Here is what that key looked like on my system:
BranchName REG_SZ fbl_impressive
ThresholdRiskLevel REG_SZ low
ThresholdOptedin REG_DWORD 0x1
10041 REG_DWORD 0x1
Notice the key RecoveredFrom and the value 10041 in it. Delete the RecoveredFrom key and do another check for updates in Windows Update. The build should now be listed.