How to merge 2 dataframes with common and also different indices, but prefer valid values against NaN values?

Question

How to combine

                     A  B    C
2010-01-01 10:00:00  1  1  1.0
2010-01-01 11:00:00  2  2  2.0
2010-01-01 12:00:00  3  3  NaN
2010-01-01 13:00:00