By using XSLT and Identity Transform pattern.
The 2nd template will find any XML element that starts with 'IMG' and transform it to just 'IMG' while keeping everyhing else as-is intact.
XML
<?xml version="1.0" encoding="UTF-8"?>
<root>
<Betalningsmottagares_x0020_postnr>401 01</Betalningsmottagares_x0020_postnr>
<Betalningsmottagares_x0020_postort>Göteborg</Betalningsmottagares_x0020_postort>
<IMG0>10</IMG0>
<IMG1>20</IMG1>
<IMG2>30</IMG2>
</root>
XSLT
<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" encoding="utf-8" indent="yes" omit-xml-declaration="no"/>
<xsl:strip-space elements="*"/>
<xsl:template match="node() | @*">
<xsl:copy>
<xsl:apply-templates select="node() | @*"/>
</xsl:copy>
</xsl:template>
<xsl:template match="*[starts-with(local-name(), 'IMG')]">
<IMG>
<xsl:value-of select="."/>
</IMG>
<xsl:apply-templates/>
</xsl:template>
</xsl:stylesheet>
Output
<?xml version='1.0' encoding='utf-8' ?>
<root>
<Betalningsmottagares_x0020_postnr>401 01</Betalningsmottagares_x0020_postnr>
<Betalningsmottagares_x0020_postort>Göteborg</Betalningsmottagares_x0020_postort>
<IMG>10</IMG>10
<IMG>20</IMG>20
<IMG>30</IMG>30
</root>