A custom dictionary is one that you can add words to or delete words from.
You can also create a custom dictionary. See this link:
http://word.mvps.org/FAQs/General/EditCustomDic.htm
If you mean that you want to change the main dictionary from US English to
UK English, follow these instructions from Word Help:
Change the default language setting:
You can change your default settings in Microsoft Office to have them match
the default settings of a different language.
1, In Microsoft Windows XP, on the Windows Start menu, point to All
Programs, point to Microsoft Office, point to Microsoft Office Tools, and
then click Microsoft Office 2003 Language Settings.
In Windows 2000, on the Windows Start menu, point to Programs, point to
Microsoft Office, point to Microsoft Office Tools, and then click Microsoft
Office 2003 Language Settings
2. Click the Enabled Languages tab.
3. In the Choose the language that defines default behavior in Microsoft
Office applications box, select the language you want, and then click OK.
4. A message appears describing the effects of the change. To continue,
click Continue and lose customizations.
