XSLT: how to rebuild a string separated by a delimiter

I have the following string R/0R4/TEST/TEST AVAILABLE/R/12345678901234C1
I want to remove 1 digit from the string after the last "/" such that 1234567890123C1
I then want to rebuild the original string with the newValue such that: R/0R4/TEST/TEST AVAILABLE/R/1234567890123C1

I have the following below which shortens the last string by 1 digit. But I do not know how to rebuild it now.

<xsl:variable name="Num14">
  <xsl:call-template name="substring-after-last">
	<xsl:with-param name="input" select="$elem/Text" />
	<xsl:with-param name="marker" select="'/'" />
  </xsl:call-template>
</xsl:variable>

<xsl:variable name="Cpn" select="substring ( $Num14, 15, 2)"/>
<xsl:variable name="Num13" select="concat ( substring ( $Num14, 1, 13), $Cpn) "/>

  <xsl:template name="substring-after-last">
    <xsl:param name="input" select="''" />
    <xsl:param name="marker" select="''" />
      <xsl:choose>
        <xsl:when test="contains($input,$marker)">
          <xsl:call-template name="substring-after-last">
            <xsl:with-param name="input" select="substring-after($input,$marker)" />
            <xsl:with-param name="marker" select="$marker" />
          </xsl:call-template>
        </xsl:when>
        <xsl:otherwise>
          <xsl:value-of select="$input" />
        </xsl:otherwise>
      </xsl:choose>
  </xsl:template>

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