How to merge two data frames based on two variables?

Question

I have 2 data frames:

year  month   value1   value2
1925    1       34       45
1925    2       56       43
....

and

year  month   value1   value2
1925    1       NA       NA
1925    2       NA       NA
....

I want to fill in the second data frame with values from the first data frame. For instance, if a row in the second data frame is year = 1925 and month = 2, then it will fill in value1 and value2 for the NA values.