Mingle help is located on the ThoughtWorks Studios website. The Mingle help files are also packaged with your local Mingle instance. In cases where network access is limited, you may want Mingle to link to your local help rather than the online help.
1. Stop MingleServer
2. Open [Mingle_Installation_Directory]/config/initializers/document.rb
3. Change HELP_DOC_DOMAIN from
HELP_DOC_DOMAIN = ONLINE_HELP_DOC_DOMAIN
HELP_DOC_DOMAIN = "/help"
The same has described in the help notes: http://www.thoughtworks.com/products/docs/mingle/13.3/help/advanced_mingle_configuration.html