Reverting from multiindex to single index dataframe in pandas

pass level=[0,1] to just reset those levels:

dist_df = dist_df.reset_index(level=[0,1])

In [28]:
df.reset_index(level=[0,1])

Out[28]:
            YEAR  MONTH  NI
datetime                     
2000-01-01  2000      1   NaN
2000-01-02  2000      1   NaN
2000-01-03  2000      1   NaN
2000-01-04  2000      1   NaN
2000-01-05  2000      1   NaN

you can pass the label names alternatively:

df.reset_index(level=['YEAR','MONTH'])

Another simple way would be to set columns for dataframe

consolidated_data.columns=country_master

ref: https://riptutorial.com/pandas/example/18695/how-to-change-multiindex-columns-to-standard-columns