Suppose I have two datasets df1 and df2 as follows:

```
df1 <- data.frame(Id=c(1L,2L,3L,4L,5L),category=c("Na","Na","Na","Na","Na"),stringsAsFactors=F);
df2 <- data.frame(Id=c(1L,3L,5L),stringsAsFactors=F)

> df1
  Id category
1  1       na
2  2       na
3  3       na
4  4       na
5  5       na
> df2
  Id
1  1
2  3
3  5

```

for those ids in df2, I would like to modify the category variable in df1 as "MC" by matching id variable. This looks so simple but having a complicated dataset, how can I do by matching id. 

The dataset should look like this.

```
> df1
  Id category
1  1       MC
2  2       na
3  3       MC
4  4       na
5  5       MC

```

Any ideas?