To change the layer for a track segment, first select it by left-clicking it and then either press either the Space key or press a number key.
Obviously it is not possible to change the track segment's layer for single sided PCB.
Pressing the space key will send the current track segment backwards in the PCB layers. However if the track segment is on the bottom layer, then it will be cycled around to the top layer. For a 2-sided board pressing the space key appears to swap a track segment to the other side of the board.
When a layer segment is swapped, vias and/or track junctions will be added and/or removed as required.
Pressing a number key will move the selected track segment to the layer defined by the number entered or to the bottom layer if the number is greater than the number of electrical layers on the PCB.
e,g, pressing '1' moves the selected track segment to the top layer while pressing '2' moved it to the second layer, in the case of a 2-layered PCB, to the bottom of the PCB.
Pressing 't' or 'T' moves the track segment to the top layer
Pressing 'b' or ''B' moves the track segment to the top layer.
You can convert a track segment to a jumper on the top or the bottom side of the board. pressing the 'H' to create a jumper on the bottom side of the board or the 'J' key to create a jumper on the top side of the board.