<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:wi="http://schemas.microsoft.com/wix/2006/wi">

  <xsl:template match="@*|node()">
    <xsl:copy>
      <xsl:apply-templates select="@*|node()" />
    </xsl:copy>
  </xsl:template>
  
  <!-- src.: https://stackoverflow.com/a/4825010/3514658 -->
  <xsl:template match="wi:ComponentGroup[@Id='MattermostDesktopFiles']/wi:Component">
    <xsl:copy>
      <xsl:attribute name="Win64">yes</xsl:attribute>
        <xsl:copy-of select="@*"/>
      <xsl:apply-templates />
    </xsl:copy>
  </xsl:template>
</xsl:stylesheet>

