I would suggest you go through the basics of XSLT in W3schools.com
You can place the code under <TokenUser></TokenUser> and <TokenPassword>pwd1</TokenPassword> as something like the following
<TokenUser>
<xsl:choose>
<xsl:when normalize-space($SystemName) = 'DEV'">
<xsl:value-of select="'User1'"></xsl:value-of>
</xsl:when>
<xsl:when normalize-space($SystemName) = 'QA'">
<xsl:value-of select="'User2'"></xsl:value-of>
</xsl:when>
<xsl:choose>
</TokenUser>
<TokenPassword>
<xsl:choose>
<xsl:when normalize-space($SystemName) = 'DEV'">
<xsl:value-of select="'pwd1'"></xsl:value-of>
</xsl:when>
<xsl:when normalize-space($SystemName) = 'QA'">
<xsl:value-of select="'pwd2'"></xsl:value-of>
</xsl:when>
<xsl:choose>
</TokenPassword>
Regards
Kalyan.