And, for very basic instructions (starting at your iPad home screen):
Go to your Settings (the gear icon on your screen). Once it opens, on the left side, you should see "Brightness & Wallpaper." Select it and, on the right hand side, you will now see some selections. Under "Wallpaper" will be two representational pictures: one of your current lockscreen picture and the other of your homescreen picture.
Select in that white area (doesn't matter which picture ... it's all the same area). You should get offered two areas from which to pick photos/pictures, "Wallpaper" (which is the stock stuff Apple sends with the iPad) and "Saved Photos," which are photos you have in your Photos app.
Select either one - and another page will open showing all the pictures you can choose. Pick one.
That picture will open up and in the right hand [upper] corner, you will see options to "Set Lock Screen," "Set Home Screen" or "Set Both." Do your selection and ta da! You've changed your wallpaper.
Oh, and on the upper left side, you will see a "Cancel" button should you decide that that particular picture is not what you want.
Just keep rinsing and repeating until you settle on a wallpaper you like/want.
Hope this helps.
Marilyn