There are multiple different ways to select multiple tracks and delete them.
The most rigourous would be: Pcbnew / Edit / Global Deletions
More precise:
Hover over a track and press “u” or “i” to select more, then press [Del]
Another basic way is to drag a box (Dragging left is different from dragging right!!!) and then [RMB] and from the popup menu click on: "Select / Filter selection to refine your selection.
While holding both [Ctrl + Shift] you can toggle entities in the selection.
This is all pretty basic stuff and I’m kind of ashamed of still not having been able to update the “Getting started in KiCad” guide for KiCad V5…