$cfg = [xml](gc  C:\Users\shish\source\web.config.txt)
$cfg.configuration.connectionStrings.add|%{
   $_.connectionString = $_.connectionString -replace "currentValue", "newValue";
   $_.name = $_.name -replace "currentValue", "newValue";
}
$cfg.Save("C:\Users\shish\source\web.config.txt");