df = pd.DataFrame([['a', 'b']], columns=['12 Mar', '12 Jun'])
df.columns.map(lambda yb: ' '.join(reversed(yb.split())))
df.columns = df.columns.map(lambda yb: ' '.join(reversed(yb.split())))
Pandas Crosstab: Change Order of Columns That Are Named as Formatted Dates (mmm yy)