I assume you mean width not copper thickness. Select the track segment then Width in the right hand panel. For more see the Tutorial.

@andyfierman I meant "thickness" and not "width", like the track thickness of a pcb coil antenna for example, but maybe it's not on EasyEDA that I should do that.

Thickness on a PCB trace refers to copper weight i.e. 1/2 oz, 1 oz, 2 oz ( I can't recall the metric dimension right now). Copper weight is a function of the PCB stack up at the time you order the board and is not specified within EasyEDA. It is in the vertical or Z direction of a board. Width refers to the width of the trace as opposed to its end to end length. Width is therefore in the XY plane of the board.

@andyfierman Ok thank you for your help!