If the subtitles you are editing have timing that is significantly off the audio track in your video, you may want to erase all the timing and sync subtitles from scratch. You can quickly and easily erase the existing timing in just a few clicks.
Click the Wrench icon above the editing panel and select Clear timing item from the Tools menu:
When prompted for a confirmation, click Continue.
This will remove all the timing, but preserve the existing subtitle boxes with the text. Next, you can display the timeline (by selecting Show Timeline item from Tools menu), play the video, and add start and end times for each subtitle by pressing Arrow Down and Arrow Up keys, respectively.