XSLT 2.0 has fn:upper-case() and fn:lower-case() functions. However in case you are using of XSLT 1.0, you can use translate():
<xsl:template match="/">
<xsl:variable name="smallcase" select="'abcdefghijklmnopqrstuvwxyz'" />
<xsl:variable name="uppercase" select="'ABCDEFGHIJKLMNOPQRSTUVWXYZ'" />
<xsl:value-of select="translate(doc, $smallcase, $uppercase)" />
</xsl:template>
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