I've made a few assumptions about the conversion logic you want...
This XSLT stylesheet:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:ns="http://graphml.graphdrawing.org/xmlns"
xmlns="http://graphml.graphdrawing.org/xmlns"
exclude-result-prefixes="ns #default">
<xsl:output method="xml"
version="1.0"
indent="yes"
omit-xml-declaration="yes"/>
<!-- The identity transform. By itself, copies out the original input. -->
<xsl:template match="node()|@*">
<xsl:copy>
<xsl:apply-templates select="node()|@*"/>
</xsl:copy>
</xsl:template>
<xsl:template match="ns:node">
<!-- Copy out this element. -->
<xsl:copy>
<!-- Give it a new 'id' attribute that takes the value of the 'data'
element for 'key' of 'd3'. -->
<xsl:attribute name="id">
<xsl:value-of select="ns:data[@key='d3']"/>
</xsl:attribute>
<!-- Output empty 'data' element for 'key' of 'dn_href'. -->
<data key="dn_href"/>
<!-- Do something particular for 'key' of 'd0'. -->
<xsl:apply-templates select="ns:data[@key='d0']"/>
<!-- Don't know where this value comes from! -->
<data key="dn_intensity_value"/>
</xsl:copy>
</xsl:template>
<xsl:template match="ns:edge">
<xsl:copy>
<!-- Add an 'id' attribute. I'll let you work out what it
should be. -->
<xsl:attribute name="id"/>
<xsl:apply-templates select="node()|@*"/>
</xsl:copy>
</xsl:template>
<xsl:template match="ns:data[@key='d0']">
<!-- It seems we want two 'data' elements, with different 'key's,
and the text from the element with 'key' of 'd0'. -->
<xsl:copy>
<xsl:attribute name="key">dn_label</xsl:attribute>
<xsl:apply-templates select="text()"/>
</xsl:copy>
<xsl:copy>
<xsl:attribute name="key">dn_free</xsl:attribute>
<xsl:apply-templates select="text()"/>
</xsl:copy>
</xsl:template>
<!-- It seems 'd5' keys get changed to 'de_strength'. -->
<xsl:template match="@key[. = 'd5']">
<xsl:attribute name="key">
<xsl:value-of select="'de_strength'"/>
</xsl:attribute>
</xsl:template>
<!-- Mapping for source attributes. -->
<xsl:template match="ns:edge/@source">
<xsl:attribute name="source">
<xsl:value-of select="//ns:node[@id=current()]/ns:data[@key='d3']"/>
</xsl:attribute>
</xsl:template>
<!-- Mapping for target attributes. -->
<xsl:template match="ns:edge/@target">
<xsl:attribute name="target">
<xsl:value-of select="//ns:node[@id=current()]/ns:data[@key='d3']"/>
</xsl:attribute>
</xsl:template>
</xsl:stylesheet>
produces the following output when applied to your example input:
<graphml xmlns="http://graphml.graphdrawing.org/xmlns"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://graphml.graphdrawing.org/xmlns http://graphml.graphdrawing.org/xmlns/1.0/graphml.xsd">
<graph edgedefault="undirected">
<node id="n1">
<data key="dn_href"/>
<data key="dn_label">rBSTS</data>
<data key="dn_free">rBSTS</data>
<data key="dn_intensity_value"/>
</node>
<node id="n2">
<data key="dn_href"/>
<data key="dn_label">rCAC</data>
<data key="dn_free">rCAC</data>
<data key="dn_intensity_value"/>
</node>
<node id="n3">
<data key="dn_href"/>
<data key="dn_label">rCMF</data>
<data key="dn_free">rCMF</data>
<data key="dn_intensity_value"/>
</node>
<edge id="" source="n1" target="n2">
<data key="de_strength">0.252829037184</data>
</edge>
<edge id="" source="n2" target="n3">
<data key="de_strength">0.205407183132</data>
</edge>
</graph>
</graphml>
As you can see, it's not perfect (extra namespaces, a few attribute values missing) but hopefully it shows you what you might do.