Running OS X Yosemite, but About This Mac shows Version 10.90
Solution 1:
Download the latest Yosemite installer from the Mac App Store but do not install it.
Copy following in to your Terminal to verify the downloaded Yosemite file is correct:
shasum /Applications/Install\ OS\ X\ Yosemite.app/Contents/SharedSupport/InstallESD.dmg
Wait for a minute for it to calculate. The result for the previous version should be:
eebf02a20ac27665a966957eec6f5e6fe3228a19 /Applications/Install OS X Yosemite.app/Contents/SharedSupport/InstallESD.dmg
And for Yosemite Download as of Nov. 27 the result is:
a673c2c6d967f4da2934b7d6cf3736936970b194 /Applications/Install OS X Yosemite.app/Contents/SharedSupport/InstallESD.dmg
Proceed with Install.
Solution 2:
Mmmh, maybe you've changed it yourself and forgotten about it?
Changing OS X version to 10.90 has been suggested to solve some compatibility issues, e.g. with Matlab (see MATLab crashes on launch OS X Yosemite).
To change it back to 10.10, type the following in a Terminal window:
sudo nano /System/Library/CoreServices/SystemVersion.plist
then change the value under 'ProductVersion' to 10.10 or whatever version you fancy.