You have no option. The beta / preview version of Office had a limited life span. It has now expired (quite a bit later than I expected, I thought it was done at the end of March). You will have to uninstall it and install the current version.
MS was quite clear, you should not have been using the Office preview for "production" work, your day to day email & calendar. It was a test lab version only. The same is true of Win "8.1" preview.
If you can still start it, I suggest you go into Outlook and export the PST. If not, copy the PST to a safe location. Then it is best to do a "clean uninstall" using method 1 in this tip:
2739501 Uninstall Office 2013 / 365 - Method 1 Fix It
http://support.microsoft.com/kb/2739501
Office 2013, Office 365 and Office 2013 Preview uninstalls
- Method 1: Uninstall using Fix it
- Method 2: Uninstall from Control Panel
- Method 3: Remove manually
Method 1 in this tip does a more thorough job that the "normal" uninstall via Programs and Features (aka Method 2 in the tip). The Normal uninstall is designed with the assumption that you will be re-installing Office. It does NOT delete
everything. It leaves configuration files like NORMAL.DOTM and other templates etc. and it also leaves configuration Registry entries.
Boot the computer. After the restart go to http://office.com/myaccount to re-install.
You can use Method 2. That will leave behind files like the PST and NORMAL.DOTM and a bunch of registry entries to make your new installation carry on from current one. Personally I prefer to clean out all "beta" code as thoroughly as possible. If possible
I prefer to re-install Windows, or better yet, to have been running the preview in a "virtual computer".