Pandas: Change day

You can use .apply and datetime.replace, eg:

import pandas as pd
from datetime import datetime

ps = pd.Series([datetime(2014, 1, 7), datetime(2014, 3, 13), datetime(2014, 6, 12)])
new = ps.apply(lambda dt: dt.replace(day=1))

Gives:

0   2014-01-01
1   2014-03-01
2   2014-06-01
dtype: datetime64[ns]