import numpy as np
import cv2

# "path"には画像のパスを入力
img = cv2.imread("path")

# BGRの順で記録されている点に注意
b = img[:, :, 0].copy()
g = img[:, :, 1].copy()
r = img[:, :, 2].copy()

# B <-> R
img[:, :, 0] = r
img[:, :, 2] = b

cv2.imshow("image", img)
cv2.waitKey(0)
cv2.destroyAllWindows()