Comparing 2 dataframes and set the value of the dataframe if not exists

Question

I have two dataframes. Examples:

df1:
    Age
0   2.0
1   NaN
2  20.0
df2:
    Age
0  10.0
1  15.0
2   NaN

Both dataframes have the same structure. What I want to do is compare each row in 2 dataframes and set the value for df1 by the value of the df2 if the value of the df1 does not exist

Any pointers would be welcome

My expected output would be:

df1:
    Age
0   2.0
1   15.0
2  20.0