Yes, you can modify the moving average lengths or any other parameter. Just modify the formulas in the Calculations worksheet and make sure the backtest worksheet parameters are set exactly the same.
Of course, the model is likely to behave differently. Backtesting will show the potential differences. If you find a combination that works better, you will need to adjust the formulas as well so the trading model and the backtest model are the same. Make sure you don’t “over-optimize” and end up curve fitting your results.