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You can always uninstall eM Client and then install the previous version you were using. You will not lose your data doing that.
You will find the full list of versions available to download at http://www.emclient.com/release-history
I am unable to remove the old version using windows. I also tried to remove it using C Cleaner.
Can you restart Windows and try again.
Go to Control Panel > Programs > Programs and Features, then right-click on the eM Client application and choose Uninstall.