How can we add new rows at the top of the CSV file using powershell script?

You mention that you want to add the new lines above the header, which means that no CSV-specific processing is needed - it sounds like you're asking how to prepend lines to an existing text file (which happens to contain CSV - note that the resulting file will no longer be a valid CSV file).

E.g., assuming a target file named some.csv:

Note: Best to make a backup of the target file before trying these commands.

If the input file is small enough to fit into memory as a whole:

Reading the entire target file into memory as a single string with Get-Content -Raw allows for a convenient and concise solution:

Set-Content -LiteralPath some.csv -NoNewLine -Value (
  @'
New line 1 above header
New line 2 above header

'@ + (Get-Content -Raw some.csv)
)

Note that Set-Content applies a default character encoding (the active ANSI code page in Windows PowerShell, UTF-8 without BOM in PowerShell Core), irrespective of the current encoding of some.csv, so you may have to use the -Encoding parameter to specify the encoding explicitly.

Also note that the single-quoted here-string (@'<newline>...<newline>'@) uses the same newline style (CRLF (Windows-style) vs. LF (Unix-style)) as the enclosing script, which may not match the style used in some.csv - though PowerShell itself has no problem processing files with mixed newlines styles.

If the file is too large to fit into memory, use a streaming (line-by-line) approach:

$ErrorActionPreference = 'Stop'

# Create a temporary file and fill it with the 2 new lines.
$tempFile = [IO.Path]::GetTempFileName()
'New line 1 above header', 'New line 2 above header' | Set-Content $tempFile

# Then append the CSV file's lines one by one.
Get-Content some.csv | Add-Content $tempFile

# If that succeeded, replace the original file.
Move-Item -Force $tempFile some.csv

Note: Use of the Get-Content, Set-Content and Add-Content cmdlets is convenient, but slow; the next section shows a faster alternative.

If performance matters, use .NET types such as [IO.File] instead:

$ErrorActionPreference = 'Stop'

# Create a temporary file...
$tempFile = [IO.Path]::GetTempFileName()

# ... and fill it with the 2 new lines.
$streamWriter = [IO.File]::CreateText($tempFile)
foreach ($lineToPrepend in 'New line 1 above header', 'New line 2 above header') {
  $streamWriter.WriteLine($lineToPrepend)
}

# Then append the CSV file's lines one by one.
foreach ($csvLine in [IO.File]::ReadLines((Convert-Path some.csv))) {
  $streamWriter.WriteLine($csvLine)
}

$streamWriter.Dispose()

# If that succeeded, replace the original file.
Move-Item -Force $tempFile some.csv