How modify the DataFrame columns

[sd_0912]

Not entirely sure what you're trying to do here, but something like this should work:

import pandas as pd
df = pd.DataFrame(columns=['Name','Category','STD'])
df = df.append({'Name': 'HAJ', 'Category': 'RT', 'STD': 'Cpl'}, ignore_index=True)
df
df.iloc[0]['STD'] = df.iloc[0]['STD'] + 'STD:JPEG'
df