To change your background, long press on your home screen, until a row of icons appear at the bottom. Select the wallpapers option.
If you haven’t used this option before, it will ask you the application you want to use to find your background;
If you use the Gallery application, this will give you images which are stored on your device.
If you have a Google account signed-in, you will have access to all your photos which are stored in your Google Photos.
You can also use third-party applications for wallpapers which may be displayed in this menu.
Select the photo that you wish to use. It will display to the size of the screen. Long press the chosen wallpaper while swiping left/right, up/down or zooming in or out to reposition it.
Once you are happy, press the select wallpaper option in the top-left corner. Your wallpaper will have updated itself.
Alternatively, you can open the folder on your home screen to access your Phone settings app. Scroll down to the Display option and select Wallpaper. It will then request which application you wish to select your wallpaper from. You can then follow the process as above (3).