If you have split an audio region but want to bring back audio that was previously there, you can use the drag tool to replace removed parts. It's also useful for editing a track without splitting it.
Use the Bracket CursorHover over the end of an audio region. You want the cursor to become a bracket sign (not an arrow grab tool or a circular loop tool).
Click and DragWhen the cursor becomes a bracket, click and drag to the right. This will bring back the part of the track that was previously there.
If you want to remove the end of the audio track, use the bracket cursor to click and drag to the left into the clip.