The shading is basically just two gradients intersecting each other. It really doesn't get much more basic than that. You can do this for yourself fairly easily. It'll probably take you longer to find and download some free editing software than it will to actually make the edit.
Also, if you don't mind my asking, where did you get that template? I've seen about a billion packs that all have that same asset in one form or another and would like to know who the original artist is.