From CauchoWiki

[edit] <server-header>

As part of the HTTP response, Resin sends its name and version number in a server header, e.g. Server: Resin/3.0.14. Some sites prefer to change that header to a different value to hide the software version that's running the site.

To enable this, in the <server> block use

[edit] resin.conf

...
<server>
  ...
  <server-header>Foo/1.0</server-header>
  ...
</server>