Hi
(i) When already routed, make a RMB click on the trace, choose properties in the popup window. There you can change the trace width.
(ii) While routing and the trace connecting is still ongoing (airwire still exist), you can enter the desired width by typing the witdth e.g. 12 (on your num-block) and than pressing enter. You will see the entered number appearing in the command line in the upper part of the program window, before pressing enter.
(iii) Above this command line, there should also be a drop down menu right next to "Width", there you can change the width as well.
(iv) For a certain area/more traces which already have been routed, mark them with the routing tool (upper left, dashed rectangle icon including a mouse courser), than press the change icon (open-end wrench) and choose the desired width. Than make a RMB click on one of the already marked/grouped traces and choose "Groupe: Change" in the poped up window.
BR