How to apply LabelEncoder for a specific column in Pandas dataframe

I have a dataset loaded by dataframe where the class label needs to be encoded using LabelEncoder from scikit-learn. The column label is the class label column which has the following classes:

[‘Standing’, ‘Walking’, ‘Running’, ‘null’]

To perform label encoding, I tried the following but it does not work. How can I fix it?

from sklearn import preprocessing
import pandas as pd

df = pd.read_csv('dataset.csv', sep=',') 
df.apply(preprocessing.LabelEncoder().fit_transform(df['label']))

You can try as following:

le = preprocessing.LabelEncoder()
df['label'] = le.fit_transform(df.label.values)

Or following would work too:

df['label'] = le.fit_transform(df['label'])

It will replace original label values in dataframe with encoded labels.