Unless you have an older 3DS (older version) and can edit your animal crossing game that way (change native fruit, grass shape, characters face, gender etc etc) there is no way to change your grass shape.
I personally love the square shaped grass though! I originally had square grass in my town and changed it to circle grass to see what the hype was about and I didn't like it... Triangles and squares give the grass a cool pattern that fits together (flat edges fit against other flat edges and allows more triangles / squares on the grass) while circles are just round blobs on the ground and don't fit together as nicely because... Round edges. IMO of course! Just love your grass if you cannot change it <3