I have a problem while using XSL.
i have an xml message which contains a tag let us say that tag is tag13.I
want to only change the value of the tag13 but print the rest of the input
as it is.
I dont know the structure of the input xml, or where and how many times the
tag13 is comming in the output.

It's a faq, see http://www.dpawson.co.uk/xsl/sect2/identity.html.
<xsl:template match="@*|node()">
	<xsl:copy>
		<xsl:apply-templates select="@*|node()"/>
	</xsl:copy>
</xsl:template>
<xsl:template match="tag13">
	<tag13>new value</tag13>
</xsl:template>
--
Oleg Tkachenko
http://www.tkachenko.com/blog
Multiconn Technologies, Israel