In Office 2013 (Preview) there’s an option to either do a side-by-side installation, or an upgrade. The side-by-side allows you to run both Office 2010 and Office 2013 on the same machine, thus allowing you to preview the new version while retaining the (presumably) working Office 2010 version.
The upgrade option allows you to do a full upgrade or simply remove one or more Office 2010 apps (such as Word or Excel) and keep the rest.
To choose between these installation types, first run the Office 2013 installer, and accept the license terms:
If you now select Upgrade, a full upgrade will be performed. We might want to avoid this if Office 2010 still holds some value for you, so select Customize:
You may now remove all previous versions (first option, a full upgrade), keep all previous versions (side-by-side with Office 2010) or remove one or more of the older Office 2010 apps:
There’s one restriction and that’s Outlook 2013. As with Outlook 2010 with Office 2010, in Office 2013 you cannot run Outlook 2013 and Outlook 2010 on the same machine. So even if you decide to go with side-by-side, you have to choose between Outlook 2010 and Outlook 2013. Otherwise you’ll end up with Office 2010 sans Outlook 2013.
To avoid this situation, select to remove Outlook 2010: