Can't change value of nodes with same name

Here is my XML file example :

<?xml version="1.0" encoding="ISO-8859-1"?>
<sc>
	<fluxs>
		<flux>
			<cible>
				<id>0</id>
			</cible>
		</flux>
		<flux>
			<cible>
				<id>1</id>
			</cible>
		</flux>
		<flux>
			<cible>
				<id>2</id>
				<wsdl_url>a</wsdl_url>
				<wsdl_url>b</wsdl_url>
			</cible>
		</flux>
	</fluxs>
</sc>

I want to change the value of nodes "wsdl_url"

I tried this :

$xml = [xml](get-content "c:\mme.xml")

# This works
write-host "Before" $xml.sc.fluxs.flux[2].cible.id
$xml.sc.fluxs.flux[2].cible.id = "Other"
write-host "After" $xml.sc.fluxs.flux[2].cible.id

# This does not work
write-host "Before" $xml.sc.fluxs.flux[2].cible.wsdl_url[0]
$xml.sc.fluxs.flux[2].cible.wsdl_url[0] = "AA"
write-host "After" $xml.sc.fluxs.flux[2].cible.wsdl_url[0]

In the first part, after having change que value of the id node, I get the new value ("Other").
In the second part, nothing change. Whats's wrong ?

As stated in comments, you can use SelectNodes method to get all wsdl_url nodes and from there is up to you what values would you like to use for each one of them. i.e. the below updates both existing nodes by index:

$nodes = $xml.SelectNodes('//cible/wsdl_url')
$nodes[0].'#text' = 'hello'
$nodes[1].'#text' = 'world'
$xml.Save('path\to\new.xml')

Then the updated XML would look like this:

<?xml version="1.0" encoding="ISO-8859-1"?>
<sc>
  <fluxs>
    <flux>
      <cible>
        <id>0</id>
      </cible>
    </flux>
    <flux>
      <cible>
        <id>1</id>
      </cible>
    </flux>
    <flux>
      <cible>
        <id>2</id>
        <wsdl_url>hello</wsdl_url>
        <wsdl_url>world</wsdl_url>
      </cible>
    </flux>
  </fluxs>
</sc>