In the Layer Sorting Table you can drag and Drop the layers to change their z-Index or Start time automatically.

Choose "By Depth" for editing the z-index of the layer/caption.

Choose "By Time" for editing the Timing of the layer/caption.

Manually Editig is also possible.  

Click on the "eye" icon to hide / show the Layer from the Drag &  Drop Section above, for easy Editing of the captions.

Click on the "lock" icon to lock / unlock the Layer from the Drag &  Drop Section above. If a layer is locked, it can't be dragged in the editor. This is useful if you have many captions and need exact positions that should not be changed anymore.

Clicking on one Layer selects that Layer for Further editing.