$xml= [xml](gc d:\111)
$xml.configuration.applicationSettings.'Vendor.Some.App.Properties.Settings'.setting
#name serializeAs value
#---- ----------- -----
#SerialConnectionTimeout String 1000
#так работает
$xml.configuration.applicationSettings.'Vendor.Some.App.Properties.Settings'.setting[2].value = '10000'
#так тоже
$node = $xml.configuration.applicationSettings.'Vendor.Some.App.Properties.Settings'.setting | ? name -eq 'SerialConnectionTimeout'
$node.value = '1111'
$xml.configuration.applicationSettings.'Vendor.Some.App.Properties.Settings'.setting[2].value
#1111
$xml.Save("d:\222")
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<applicationSettings>
<Vendor.Some.App.Properties.Settings>
<setting name="TimeoutStep" serializeAs="String">
<value>300</value>
</setting>
<setting name="TimeoutDuration" serializeAs="String">
<value>5000</value>
</setting>
<setting name="SerialConnectionTimeout" serializeAs="String">
<value>1111</value>
</setting>
</Vendor.Some.App.Properties.Settings>
</applicationSettings>
</configuration>
Кроме того: