DigitalVolcano Software Support

example:

﹂main folder
----﹂a1
--------﹂1.jpg
--------﹂2.jpg
----﹂a2
--------﹂1.jpg
--------﹂2.jpg
--------﹂3.jpg
----﹂b1
--------﹂x.mp4
--------﹂y.mp4
----﹂b2
--------﹂x.mp4
--------﹂y.mp4
--------﹂z.mp4

How to delete a1 and b1 without changing a2 and b2?

I don't think you can do this automatically.
You'd have to go in and mark the folders manually (you can right click to get a menu for folder marking).