There are two different ways you can modify an existing backtest.
Inside of every backtest is an “Edit” button.

You can select the button to access the inputs and edit the data. You can then re-run the backtest with the new values to see the different results.

Editing only the capital allocation demonstrates how the strategy will perform with different amounts of capital while keeping the strategy the same. The backtest can perform this function very quickly because none of the strategy’s details have changed.
You can also modify the backtest and use different inputs for the trade. You can change the strategy type, the components within the strategy, and/or the entry and exit conditions. This will take longer than changing only the capital allocation because the backtester must compile all new historical data.
NOTE: Modifying an existing backtest replaces that version. If you want to preserve the original results, simply create a new backtest.