The initial clearance value will be taken from the DRC dialog box (trace to trace). Select two parallel segments, between which there are other segments of other trace (Note: If you want the bottom segment to remain in place, then select the top one and then the bottom):
The clearance value is shown in the status bar at the bottom of the program window. You can then change the clearance by pressing the
F3 buttons. The modified clearance value will be stored in FreePcb-2 memory until you press the file save button at the top left.