In About this Mac it shows that the OS is Yosemite but the version number is 10.90. Is this normal? I thought Yosemite is 10.10, and Mavericks was 10.9.
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.
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.