How do I get Heirloom’s original values back after changing a row?

In two clicks, so you can try your own numbers without risk. Click the three dots at the end of the row, then Reset to default value: your values are removed and Heirloom’s come back.

Delete row is for rows with no Heirloom values behind them, such as a crop or cultivar you created yourself. Plantings already planned keep their values either way; to send the restored values to them, use Apply to existing plantings.

The steps are in Change your crop and cultivar values.