Update data frame via function doesn't work

test in your function is a copy of the object from your global environment (I'm assuming that's where it is defined). Assignment happens in the current environment unless specified otherwise, so any changes that happen inside the function apply only to the copy inside the function, not the object in your global environment.

And it's good form to pass all necessary objects as arguments to the function.

Personally, I would return(test) at the end of your function and make the assignment outside of the function, but I'm not sure if you can do this in your actual situation.

test.fun <- function (x, test) {
    test[test$v1==x,"v2"] <- 10
    return(test)
}
test <- data.frame(v1=c(rep(1,3),rep(2,3)),v2=0)
(test <- test.fun(1, test))
#  v1 v2
#1  1 10
#2  1 10
#3  1 10
#4  2  0
#5  2  0
#6  2  0

If it is absolutely necessary to modify an object outside your function directly, so you need to tell R that you want to assign the local copy of test to the test in the .GlobalEnv.

test.fun <- function (x, test) {
    test[test$v1==x,"v2"] <- 10
    assign('test',test,envir=.GlobalEnv)
    #test <<- test  # This also works, but the above is more explicit.
}
(test.fun(1, test))
#  v1 v2
#1  1 10
#2  1 10
#3  1 10
#4  2  0
#5  2  0
#6  2  0

Using assign or <<- in this fashion is fairly uncommon, though, and many experienced R programmers will recommend against it.

Changing the <- to <<- in your function, does the trick as well, see the R-manual . Quote from that page:

The operators <<- and ->> are normally only used in functions, and cause a search to made through parent environments for an existing definition of the variable being assigned. If such a variable is found (and its binding is not locked) then its value is redefined, otherwise assignment takes place in the global environment.

Your code should then be:

test <- data.frame(v1=c(rep(1,3),rep(2,3)),v2=0) 

test.fun <- function (x) {
  test[test$v1==x,"v2"] <<- 10
  print(test)
}

test.fun(1)