How to update a value of a cell of a dataframe using the value of two cells below?

I have the following dataframe (created for an example):

In [2]: df
Out[2]:
   A  B
0  1  2
1  1  3
2  4  6

Assuming that this is a long dataframe, whenever there is a value of 2 in row B, I need to replace that value with the value that is two cells below that.

I tried:

for val in df['B']:
    if val == 2:
        df['B'] = df.B.shift(-2)

But I get:

    A   B
0   1   6.0
1   1   NaN
2   4   NaN

And I need:

Out[2]:
   A  B
0  1  6
1  1  3
2  4  6

Does anyone have suggestions for leaving rows that do not equal ‘2’ unchanged?

Answer

Then we just need to assign it with shift

df.loc[df['B'].eq(2),'B'] = df.B.shift(-2)
df
Out[458]: 
   A    B
0  1  6.0
1  1  3.0
2  4  6.0