R: merge two tables when one table is a special case of the another one

I want to merge two table with different row numbers by the specific way.
I have the next:

df1 <- data.frame(num = c(1,1,1,2,2,2),
                  lab = c("A", "B", "C", "A", "B", "C"),
                  val = c(0,0,0,0,0,0))
df1
  num lab val
1   1   A   0
2   1   B   0
3   1   C   0
4   2   A   0
5   2   B   0
6   2   C   0

df2 <- data.frame(num = c(1,1,2),
                  lab = c("A", "B", "A"),
                  val = c(10,10,10))
df2
  num lab val
1   1   A  10
2   1   B  10
3   2   A  10

I want to merge df1 and df2 to get df3:

df3 <- data.frame(num = c(1,1,1,2,2,2),
                  lab = c("A", "B", "C", "A", "B", "C"),
                  val = c(10,10,0,10,0,0))
df3
  num lab val
1   1   A  10
2   1   B  10
3   1   C   0
4   2   A  10
5   2   B   0
6   2   C   0

How to do that?

>Solution :

You can do it with one line:

dplyr::rows_update(df1, df2, by = c("num", "lab"))
#   num lab val
# 1   1   A  10
# 2   1   B  10
# 3   1   C   0
# 4   2   A  10
# 5   2   B   0
# 6   2   C   0