You can use the following xPath to get the node with Group Name=OSD Value key=3:
//OSDCollBuild/Group[@Name='OSD']/Value[@key='3']
Here is a full example to change the text value:
$xml = [xml]@"
<OSDCollBuild>
<Group Name="OSD">
<Value key="1">1809</Value>
<Value key="2">1909</Value>
<Value key="3">1909.1</Value>
<Value key="4">20H2</Value>
</Group>
<Group Name="Office">
<Value key="1">Standard</Value>
<Value key="2">Professional</Value>
<Value key="3">Student</Value>
</Group>
</OSDCollBuild>
"@
# find the node to change
$node = $xml.SelectSingleNode("//OSDCollBuild/Group[@Name='OSD']/Value[@key='3']")
# change the value
$node.InnerText = "1909.2"
# dump the xml to verify the change
$xml.OuterXml
Output:
<OSDCollBuild>
<Group Name="OSD">
<Value key="1">1809</Value>
<Value key="2">1909</Value>
<Value key="3">1909.2</Value>
<Value key="4">20H2</Value>
</Group>
<Group Name="Office">
<Value key="1">Standard</Value>
<Value key="2">Professional</Value>
<Value key="3">Student</Value>
</Group>
</OSDCollBuild>
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