[trouble installing repaints]

I understand the steps to installing a repaint but i am having trouble following one of the steps.

its adding the text i need to add to the aircraft.cfg

heres what i do i add the text change the number like im suppose to but then i try to save it i get this popup

aircraft.cfg already exists would you like to replace it so i click ok then i get this

access denied.

im stumped what can i do to fix this problem thanks in advance.

oh if it helps im using windows 7