In case you are trying to update Cyanogenmod the a current Nightly built (e.g. Nightly Cm-11-20150201) chances are that the CM updater crashes before even rebooting to the actual updating process. This problem has been confirmed by PsychoI3oy on the CM forum:
Yeah, we’ve been getting a lot of crashes in the backend from cmupdater. It has been fixed and we’re doing a special re-run of cm11 nightlies starting today that will have this fix. You’ll still have to manually install this update but after that it should be fine. For anyone else having this issue: please stop hitting report on the crash button. We have plenty of reports. Thanks
The solution is pretty simple: Just do a manual update by powering up your device while pressing Home + Volume Up + Power simulateously. Of course make sure to download a recent nightly version using CM updater. As stated in various bug reports this issue will be fixed once a manual update was executed. Personally, I did not experience problems with updates using the regular CM updater approach after executing the process manually as described above.