Why this upper column dataframe function doesn't work?

Question

I want to create a function to upper columns, I already try this:

def upperColumn(DataframeColumn):
  DataframeColumn = DataframeColumn.str.upper()

upperColumn(df['ANYTHING'])

I need this to be the same as:

df['ANYTHING'] = df['ANYTHING'].str.upper()

obs. Pandas is imported and the dataframe is created already.

Thank you in advance.