To change the default delimiter of Mustache.js, you can use the Mustache.tags property. By default, Mustache.js uses {{ and }} as delimiters for variables and sections. To change these delimiters, you can set the Mustache.tags property to an array containing the new opening and closing delimiters. For example, if you want to use [[ and ]] as delimiters, you can set Mustache.tags to ['[[', ']]']. This will change the default delimiters used by Mustache.