To add a new column

df['newcolumn']='new' + df['col1']

To remove a column

del df[df.columns[2]]   <<aka>>   del df['newcolumn']

Result

Result

To change column data type

df[['col1']] = df[['col1']].astype(str)

Result

To rename column

df.rename(columns={'col1':'column1'})

Result