Pandas: How can I use the apply() function for a single column?

I have a pandas data frame with two columns. I need to change the values of the first column without affecting the second one and get back the whole data frame with just first column values changed. How can I do that using apply in pandas?

Given a sample dataframe df as:

    a,b
    1,2
    2,3
    3,4
    4,5

what you want is:

    df['a'] = df['a'].apply(lambda x: x + 1)

that returns:

       a  b
    0  2  2
    1  3  3
    2  4  4
    3  5  5

From: stackoverflow.com/q/34962104