What you could do is, extract sub strings, and re-join them as needed.
<date><xsl:value-of select="concat(substring-after(substring-after(date/text(),'/'),'/'), '-', substring-before(date/text(),'/'), '-', substring-before(substring-after(date/text(),'/'),'/'))" /></date>
or better use a template for this:
<xsl:template name="format_date">
<xsl:param name="date" />
<xsl:value-of select="concat( substring($date, 7, 2),'.',substring($date, 5, 2), '.', substring($date, 1, 4), ', ', substring($date, 9, 2),':',substring($date, 11, 2),'h' )" />
</xsl:template>
and pass the date as param (with-param).
which'll give you the desired <date>2020-08-26</date>