Use the Clear All Data feature in Flow History

At the top right of the Flow History pane is the option Clear All Data.

The Clear All Data button will delete all execution data that has been collected for the flow. You'll still see the list of flow executions that shows the success or failure of each execution as well as the date and time of each execution, but the data that passed through the flow will no longer be available and you won't be able to use the Replay option for a failed execution.

If you no longer want to save data for the future executions, click Clear All Data. You will have the option to stop saving data or continuing to save data for future executions.

For more information on saving data, see Save data with Flow History.