Я нашел два пути:
df.index = df.index.astype(str).str.replace('low', 'down')
df
NUM Npc Value
OOF_Mic.Y9_down_of_XX 0.420697 Nil 0.000666
OOF_Mic.Y5_down_of_XX 0.420697 Nil 0.000666
OOF_Mic.Y7_down_of_XX 0.420697 Nil 0.000666
или
df1 = df.reset_index().rename(columns = {'index':'temp'})
df1.loc[:,'temp'] = df1.temp.astype(str).str.replace('low', 'down')
df1.set_index('temp')
NUM Npc Value
temp
OOF_Mic.Y9_down_of_XX 0.420697 Nil 0.000666
OOF_Mic.Y5_down_of_XX 0.420697 Nil 0.000666
OOF_Mic.Y7_down_of_XX 0.420697 Nil 0.000666