df1 before update: 
    A  B  C
0  1  4  7
1  2  5  8
2  3  6  9
df2: 
     B
0  10
1  11
2  12
df1 after update: 
    A   B  C
0  1  10  7
1  2  11  8
2  3  12  9

Please note that if any of the values of column B of df2 is nan, then that corresponding value in df1 will not get updated.

import numpy
import pandas

df1 = pandas.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6], 'C': [7, 8, 9]})
print("df1 before update: \n", df1)

df3 = pandas.DataFrame({'B': [13, numpy.nan, 15]})
df1.update(df3)
print("df1 after update: \n", df1)

The above program will print the following output:

df1 before update: 
    A  B  C
0  1  4  7
1  2  5  8
2  3  6  9
df1 after update: 
    A     B  C
0  1  13.0  7
1  2   5.0  8
2  3  15.0  9