in.txt 내용이 아래와 같을때 1을 0 으로, 0을 1로 바꾸고 싶습니다.

arr[i][j].replace('0','1') 가 동작하지 않는데 어떻게 하면 될까요?

in.txt

0000
1111
0101
1010

python code

num = 4
arr = [[0]*num for i in range(num)]

with open("in.txt","r") as f:
    for i in range(num):
        arr[i] = f.readline().strip()

for i in range(num):
    for j in range(num):
        if arr[i][j] == '0':
            arr[i][j].replace('0','1')
        else:
            arr[i][j].replace('1','0')

with open("out.txt","w") as f:
    for line in arr:
        f.write(line)
        f.write('\n')