How to remove timezone from a Timestamp column in a pandas dataframe

The column must be a datetime dtype, for example after using pd.to_datetime.
Then, you can use tz_localize to change the time zone, a naive timestamp corresponds to time zone None:

testdata['time'].dt.tz_localize(None)

Unless the column is an index (DatetimeIndex), the .dt accessor must be used to access pandas datetime functions.