Layer toggle in kicad
In the PNS router, before you begin placing a new track, press the v key to toggle between your paired layers.  Correspondingly, after you begin laying the track, v will place a via (and switch you to the paired layer).

If you wanted to switch to a specific layer, the default hotkeys are:


Top Copper -- PgUp
Bottom Copper -- PgDown
Inner Layer 1 -- F5
Inner Layer 2 -- F6
Inner Layer 3 -- F7
Inner Layer 4 -- F8
Inner Layer 5 -- Shift-F5
Inner Layer 6 -- Shift-F6


You can shift up and down layers with + and -