how do i change my back ground

i have a back ground i do not like and i cant find the files so i can delete it cuz i dont know what there called!
And its in a folder but i cant find IT i am tryin to put a fallout 3 back ground in can any one help :slight_smile:
heres the back ground i got that i wanna get rid of: http://www.garrysmod.org/downloads/?a=view&id=26612

just try installing the new background and overwriting any files it asks to overwrite