Hi, if you need to take off the bumper, check out this link:
Or you may be able to get at the bulb via the wheel arch: http://www.carcarekiosk.com/video/2008_Subaru_Impreza_2.5i_2.5L_4_Cyl._Sedan/lights/turn_signal_front
The second link shows how to change indicator bulb, but you may be able to do side lights this way maybe?
Hope this helps