I'm going to assume you mean wallpaper. There's two easy ways to do this:
Press and hold a blank area of the screen, and it'll go into "edit mode", with four options in a box at the bottom of the screen; Widgets, Shortcuts, Folders and Wallpapers. Press Wallpapers.
Or press the Menu button to the left of the Home button which brings up 6 options; Add, Wallpaper, Search, Notifications, Edit and Settings. Choose Wallpaper.
Happy editing!