In [2]:
df
Out[2]:
A B C
0 0 0 a
1 0 1 b
2 1 2 c
3 1 3 d
4 2 4 e
5 2 5 f

Replace values that meet a condition with the values in another column.

In [3]:
df['A'].mask(df['A'] == 1, df['B'], axis=0, inplace=True)
df
Out[3]:
A B C
0 0 0 a
1 0 1 b
2 2 2 c
3 3 3 d
4 2 4 e
5 2 5 f

Replace values that meet a condition with a function value.

In [5]:
df['A'].mask(lambda x: x > 1, lambda x: x + 10, inplace=True)
df
Out[5]:
A B C
0 0 0 a
1 0 1 b
2 1 2 c
3 1 3 d
4 12 4 e
5 12 5 f