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To change a group, highlight the group, click on the change tool, select width, scroll to the width you want and select it. Then right click the board and click change group. If the width you want is not in the dropdown menu, click on the three dots at the bottom of the dropdown menu. A dialog will appear, and you can enter any width within the allowed limits there.
With my version of Eagle, 0.6 inch in an invalid width. The widest is 0.5xx inches. 0.6 mm or cm should be valid choices. A little trick for a single section to avoid using the command line or change dialog is to select the information icon, then enter a value that you want for the line width.