How to edit the system/user dictionary in Windows 8?

This info might help, not sure what your skill set is or what roadblocks you may run into trying to edit the file.

Source

The user-specific dictionaries for a language, which hold the content for the Added, Excluded, and AutoCorrect word lists, are located under %AppData%\Microsoft\Spelling\. The filenames are default.dic (Added), default.exc (Excluded) and default.acl (AutoCorrect). The files are UTF-16 LE plaintext that must start with the appropriate Byte Order Mark (BOM). Each line contains a word (in the Added and Excluded word lists), or an autocorrect pair with the words separated by a vertical bar ("|") (in the AutoCorrect word list). Other .dic, .exc, and .acl files present in the directory will be detected by the spell checking service and added to the user word lists. These files are considered to be read-only and are not modified by the spell checking API.

I would have just written a comment, but I don't have enough reputation (yet)!

The answer (while aimed at an API) is correct, although the comment about read-only only applies to the API, and not to simply editing the files with a text editor.

The second answer is not quite so wrong as the commenter might think. It is true that Office & Windows spell checking are separate entities, and it's not possible to change the locations for the Windows spell checker. And - as far as I can find out - there is no way to edit other than text editing the files mentioned in the accepted answer.

However, if Office 2013 is installed on a clean install of Windows 8.1, Office will in fact use the custom dictionaries already in Windows for the language-specific dictionaries.

That means the second answer stating that you can edit the dictionaries in options of an Office program is actually correct... assuming Windows was not upgraded over the Office installation.