If you want to update the declared resource, you need to make use of the resource reference which is denoted by the Capital letter of the type reference. The following example is found in the Puppet 3 Reference Menu.

file {'/etc/passwd':
  ensure => file,
}

File['/etc/passwd'] {
  owner => 'root',
  group => 'root',
  mode  => 0640,
}

Done =)

Reference: Puppet 3 Reference Menu – Amending Attributes With a Reference