When you are editing a subtitle set, you may want to erase and re-type the entire text - for example, if you are seeing a poor quality translation. You can quickly and easily erase the existing text in just a few clicks.
Click the Wrench icon above the editing panel and select Clear text item from the Tools menu:
When prompted for a confirmation, click Continue.
This will remove all the subtitle text, but preserve the existing subtitle boxes and timing. You can type or paste new text into these boxes.