df

	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.

df['A'].mask(df['A'] == 1, df['B'], axis=0, inplace=True)
df

	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.

df['A'].mask(lambda x: x > 1, lambda x: x + 10, inplace=True)
df

	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