Hi
Any help is greatly appreciated!!!

How can I go through my xml file and replace the following with subscript: ^-15

Use the generic copy template and augment with your specific
rules:

 <xsl:stylesheet version="1.0"  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="node()|@*">
    <xsl:copy>
      <xsl:apply-templates select="node()|@*"/>
    </xsl:copy>
  </xsl:template>

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