Here is one way of doing it.
<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml"/>
<xsl:template match="Employee">
<Employee>
<xsl:apply-templates select="EmployeeDetails" />
</Employee>
</xsl:template>
<xsl:template match="EmployeeDetails">
<xsl:variable name="employee" select="@EmployeeName" />
<xsl:variable name="id" select="@EmployeeId" />
<xsl:element name="EmployeeDetails">
<xsl:attribute name="{$employee}"><xsl:value-of select="$id" /></xsl:attribute>
<xsl:attribute name="CompanyName"><xsl:value-of select="@CompanyName" /></xsl:attribute>
<xsl:attribute name="CompanyAddress"><xsl:value-of select="@CompanyAddress" /></xsl:attribute>
<xsl:attribute name="ContactNo"><xsl:value-of select="@ContactNo" /></xsl:attribute>
</xsl:element>
</xsl:template>
</xsl:stylesheet>