how to Merge two xml files with XSLT

Pretty much the same answer as I provided to your last question, modified to match your new XML format:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" indent="yes"/>
  <xsl:param name="fileName" select="'updates.xml'" />
  <xsl:param name="updates" select="document($fileName)" />

  <xsl:variable name="updateItems" select="$updates/feed/entry" />

  <xsl:template match="@* | node()">
    <xsl:copy>
      <xsl:apply-templates select="@* | node()"/>
    </xsl:copy>
  </xsl:template>

  <xsl:template match="feed">
    <xsl:copy>
      <xsl:apply-templates select="@* | node()[not(self::entry)] | 
                                   entry[not(id = $updateItems/id)]" />
      <xsl:apply-templates select="$updateItems" />
    </xsl:copy>
  </xsl:template>
</xsl:stylesheet>

When run on the first sample XML, with the second one saved as "updates.xml", this produces:

<feed>
  <author>
    <firstName>f</firstName>
    <lastName>l</lastName>
  </author>
  <date>2011-01-02 </date>
  <entry>
    <id>1</id>
    <Name>aaa</Name>
    <Content>XXX</Content>
  </entry>
  <entry>
    <id>2</id>
    <Name>newName</Name>
    <Content>newContent</Content>
  </entry>
  <entry>
    <id>3</id>
    <Name>ccc</Name>
    <Content>ZZZ</Content>
  </entry>
</feed>