(Get-Content c:temptest.txt).replace('[MYID]', 'MyValue') | Set-Content c:temptest.txt

(Get-Content c:temptest.txt) -replace '[MYID]', 'MyValue' | Set-Content c:temptest.txt

(Get-Content file.txt) |
Foreach-Object {$_ -replace '[MYID]','MyValue'}  |
Out-File file.txt

$content = [System.IO.File]::ReadAllText("c:bla.txt").Replace("[MYID]","MyValue")
[System.IO.File]::WriteAllText("c:bla.txt", $content)

$path = "C:testFile.txt"
$word = "searchword"
$replacement = "ReplacementText"
$text = get-content $path
$newText = $text -replace $word,$replacement
$newText > $path

get-content $pathToFile | % { $_ -replace $stringToReplace, $replaceWith } | set-content $pathToFile

try
{
   $reader = [System.IO.StreamReader] $pathToFile
   $data = $reader.ReadToEnd()
   $reader.close()
}
finally
{
   if ($reader -ne $null)
   {
       $reader.dispose()
   }
}

$data = $data -replace $stringToReplace, $replaceWith

try
{
   $writer = [System.IO.StreamWriter] $pathToFile
   $writer.write($data)
   $writer.close()
}
finally
{
   if ($writer -ne $null)
   {
       $writer.dispose()
   }
}

Get-ChildItem 'C:yourfile*.xml' -Recurse | ForEach {
     (Get-Content $_ | ForEach  { $_ -replace '[MYID]', 'MyValue' }) |
     Set-Content $_
}