Nirvana of Windows 8 is compelling most of the Windows OS users to have a spin for Windows 8.1 but many are worried about loosing data, files and most importantly the apps installed. Though Microsoft documentation says that  you cannot upgrade to "Windows 8.1” without loosing your apps as the update only gives “Keep personal files only” option but there is a way around.

In principle upgrading Windows 8 to 8.1 without loosing app data, requires that the installation's language must match your system's default UI language and a specific Windows 8 edition can only be upgraded to its equivalent or higher edition.

For example, if you have Windows 8 Pro English-GB installed, then you can upgrade to Windows 8.1 Pro English-GB without compromising application data.

Depending upon which Windows 8 edition you have currently installed,  there are three possible cases. If you wish to upgrade to

Windows 8.1

You can only upgrade to this version if you have Windows 8 installed (not Windows 8 Pro or Enterprise).

Windows 8.1 Pro

You can only upgrade to this version, while keeping personal files and apps intact,  if you have equivalent or lower edition of Windows 8 installed, which are:

1) Windows 8
2) Windows 8 Pro
3) Windows 8 Pro with Media Center.

Windows 8.1 Enterprise

You can only upgrade to this version, while keeping personal files and apps intact,  if you have equivalent or lower edition of Windows 8 installed, which are:

1) Windows 8
2) Windows 8 Pro
3) Windows 8 Pro with Media Center.
4) Windows 8 Enterprise edition.

Note: If the option to keep "personal files and apps" doesn't appear under "Choose what to keep" menu, than make sure that the language and the product key of current installation and what you are about to install matches.