Hi sapphire,
if you want to highlight the net select DISPLAY--->Assign color and then select (right hand side) click find to select the which one going to highlight like net ,symbol ,pin etc and select the color from option buttton.
if you want to change 10 mils to 8 mils trace first select setup---> constraints----> physical work space of constraints will open and then select net layer change the trace width one by one (ensure which net going to change )
or
Delete which net going to change and then select line width 8mils and then route it