To delete a backtest, first go to the strategy detail then click on the backtest you wish to remove. In the top-righ corner you will see a menu button where you can select 'Delete'.
Deleting a backtest only removes the record it created for that version (i.e. any results which were created by executing the strategy on historical data with certain parameters assigned). It does not affect any deployment that was launched from that backtest version.
Backtests are shown publicly if the deployment is published to the Strategies Marketplace. If you do not want the backtest to be published along with the strategy, you need to delete the backtest record.
Deleting a backtest for an already published strategy will remove backtest records from the published strategy detail as well. If you are sure you want to remove it, confirm your choice in the pop-up window.