$filePath = 'c:\Users\MyName\Desktop\PLC_Comm_D13_VCL\uMain.pas'

$oldCode = @"
procedure TfMain.OnMQTTMessage(const ATopic, APayload: string);
begin
//  mmMQTTmsg.Lines.Add(ATopic+'   '+APayload);
  Edit2.Text := ATopic+'   '+APayload;
end;
"@

$newCode = @"
procedure TfMain.OnMQTTMessage(const ATopic, APayload: string);
var
  JsonObj: TJSONObject;
  i, DONum: Integer;
  Pair: TJSONPair;
  KeyStr, ValStr: string;
begin
//  mmMQTTmsg.Lines.Add(ATopic+'   '+APayload);
  Edit2.Text := ATopic+'   '+APayload;

  try
    JsonObj := TJSONObject.ParseJSONValue(APayload) as TJSONObject;
    if Assigned(JsonObj) then
    begin
      try
        for i := 0 to JsonObj.Count - 1 do
        begin
          Pair := JsonObj.Pairs[i];
          KeyStr := Pair.JsonString.Value;
          ValStr := Pair.JsonValue.Value;

          if (Length(KeyStr) >= 5) and (Copy(KeyStr, 1, 3) = 'DO_') then
          begin
            DONum := StrToIntDef(Copy(KeyStr, 4, Length(KeyStr) - 3), -1);
            if DONum > 0 then
            begin
              if ValStr = '1' then
                SendModbus(slvID, cmdFSC, 15 + DONum, 1)
              else if ValStr = '0' then
                SendModbus(slvID, cmdFSC, 15 + DONum, 0);
            end;
          end;
        end;
      finally
        JsonObj.Free;
      end;
    end;
  except
    on E: Exception do printlog('MQTT Msg Error: ' + E.Message);
  end;
end;
"@

$content = Get-Content $filePath -Encoding Default -Raw

$content = $content.Replace($oldCode, $newCode)

Set-Content -Path $filePath -Value $content -Encoding Default