How to change the value of XML Element attribute using PowerShell?

You can access the attributes directly in the [xml] object like this:

# C:\temp> $xml = [xml](Get-Content C:\FE6Work.xml)
# C:\temp> $xml.office.staff

branch                   Type                           employee                                                             
------                   ----                           --------                                                             
Hanover                  sales                          {Tobias Weltner, Cofi Heidecke}                                      
London                   Technology                     {XXXX, Cofi}                                                         

# C:\temp> $xml.office.staff | foreach{$_.branch = "New York"}
# C:\temp> $xml.office.staff

branch                   Type                           employee                                                             
------                   ----                           --------                                                             
New York                 sales                          {Tobias Weltner, Cofi Heidecke}                                      
New York                 Technology                     {XXXX, Cofi}                                                         

if we are taking attribute from console and changing its value ?

$path=Read-Host -Prompt 'Enter path of xml file'
[xml]$xmldata = get-content "$path"

$tag = Read-Host -Prompt 'Enter tag'
$value = Read-Host -Prompt 'Enter value'
$xmldata.InstallConfig.$tag="$value"
$xmldata.Save($path)

Try the following:

$nodes = $xml.SelectNodes("/office/staff");
foreach($node in $nodes) {
    $node.SetAttribute("branch", "New York");
}

This will iterate through all nodes returned by SelectNodes() and modify each one.