Change value of an XML attribute via xslt -

i have following xml-file:

<book description="for beginners" name="it book">     <available>yes</available>     <info pages="500.</info> </book> 

i want this:

<book description="for pros" name="it book">     <available>yes</available>     <info pages="500.</info> </book> 

i looked how modify xml-documents on internet. found out first of should declare template copying everything:

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

however, dont know how write template actual modification. helping out beginner.

edit: here stylesheet far (as requested ul1):

<?xml version="1.0" encoding="utf-8"?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/xsl/transform" xmlns:sig="http://www.w3.org/2000/09/xmldsig#">     <xsl:output indent="yes" method="xml" omit-xml-declaration="yes"/>      <xsl:template match="node() | @*">         <xsl:copy>             <xsl:apply-templates select="node() | @*"/>         </xsl:copy>     </xsl:template>      <xsl:template match="@description='for beginners'">         <xsl:attribute name="{name()}">             <xsl:text>for pros</xsl:text>         </xsl:attribute>     </xsl:template> </xsl:stylesheet> 

this question answered in many other threads. eg. xslt: how change attribute value during <xsl:copy>?

in case, need template, matches on attribute description, besides identity-copy template.

<xsl:template match="@description"> <!-- @ matches on attributes, possible restrict! -->   <xsl:attribute name="{name()}">   <!-- creates new attribute same name -->     <xsl:text>for pros</xsl:text>   <!-- variable statement desired value -->   </xsl:attribute> </xsl:template> 

edit 1 (further information cause of errors)

one complete, valid, runnable script be:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/xsl/transform" version="1.0">    <xsl:template match="@* | node()">     <xsl:copy>       <xsl:apply-templates select="@* | node()"/>     </xsl:copy>   </xsl:template>    <xsl:template match="@description[. = 'for beginners']">     <xsl:attribute name="{name()}">       <xsl:text>for pros</xsl:text>     </xsl:attribute>   </xsl:template>  </xsl:stylesheet>