How to use 'vias' to let tracks 'cross'
Many simple boards can be completed with tracks on only one side of the PCB.
When just a few tracks have to "cross", money can be saved by using wire "links" on the upper side of the board.
As boards become more complex, the number of links needed becomes so large that it is worth paying for a double sided board with through plated holes.
Whether you are using wire links, or tracks on the upper side of the board, during your work in PCBnew, you will use the same "answer".
Suppose you were making a board, and you've got as far as….
…. and you want to connect "A1" and "B1", to satisfy the rat's nest line you see. (And there's no clever way to "go around" the already laid down tracks we see as vertical green lines.)
Click on the "Add tracks and vias" button on the right hand tool bar.
Right-click in the working area, and select "Select Layer Pair for Vias". The following should pop up….
Those are the default settings, good settings. I've only mentioned it in case you've Done Something to upset them.
Now click on the "Add tracks and vias" button on the right hand tool bar. Click on "A1", to start a track there, move a bit to the right, press "V". This will create a via in the line. Move a bit more to the right, and you will find that you are still laying down a track, but now it is in red. This is to signify that it is on the top side of the board. Keep going right until you have crossed over the two vertical green lines, the tracks you needed to cross. Press "V" again. Keep moving to the right, until you reach "B1", then double click. That's it! You've laid down a track which starts in the back side of the board, drawn in green. Goes "through" the board by means of ("via"!) the via. Continues on the top of the board. Goes back to the back side of the board… and finishes at "B1".
That same PCB design is fine whether you want to make a double sided board, in which case the red line is a track on the front side, or whether you want to make a single sided board, in which case the red line shows you where to put a wire link.
I don't believe there is a simple way to "mark" a section of existing track and "move" it to a different layer…. Just delete tracks that need to be done with vias, and re-lay them, pressing "V" as needed as you proceed.