SystemTrader allows you to edit the historical prices and trading volumes of any stock ticker.
In the list-view, click on the stock ticker you want to edit.
Choose Tickers > Edit historical data. The Edit Ticker dialog appears.
Click on the cell you want to edit, then click on it again to open the editor. Enter a new value then press ENTER.
You can also process a stock-split by clicking Add Split.
Click Save to save the new prices and volumes.