Auto Attacks and Movement

I like to play adc and from time to time i have a stupid problem where in the heat of battle i missclick next to the target i want to attack and instead of attacking him i start walking towards him... so i checked in the options and i wanted to change so when i left click(MB1) i will attack and when i right click(MB2) i will move but when i press to change it i get a box where i have to press the button witch i want to change to.. but when i left click (MB1) it disappears. SOO .. ;d If anyone can tell me is it possible to change it or it is a bug.. sorry for the bad spelling and thank you!
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