Unfortunately this is not so easy, due to the problem that the update mechanism with files has an issue as mentioned here.
The easiest way is probably what written here by @MarkWilliamMatthews:
It is also possible to provide your own local repository by copying all files from the plugin repository to a local directory. This directory needs to be added to the list of repositories.
You can configure such a repository by by adding a properties file to the configuration.
On Unix this file needs to be placed at
%userprofile%\.netbeans\7.1\config\Preferences\org\netbeans\modules\autoupdate\{Name_Of_Provider}.properties
On Windows at:
%userdir%\AppData\Roaming\SNAP\config\Preferences\org\netbeans\modules\autoupdate\{Name_Of_Provider}.properties
This is an example file:
local.properties (202 Bytes)
When you have added the repositories this way, you can do the update following the guide
Update SNAP from the command line - SNAP - SNAP Wiki (atlassian.net)