I would write an identity-translate XSL and then apply that XSL to all the XMLs in that folder using whatever batch technology you wish (.bat, VB app if you like). Write a match for the specific element or elements you wish to change and then include a general template that outputs all the content of everything else as is.
Without testing, it should be something like this:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:template match="//Valid/HeaderInfo/SoftwareID">
<SoftwareID>
<xsl:text>53623745</xsl:text>
</SoftwareID>
</xsl:template>
<xsl:template match="@*|node()">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
You could even expand that example to pass in the new value as a parameter into the transform so you never have to even edit it.