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In order to assign a variable to the global environment from within a function, there are two ways to proceed: either using the assign operator `<<-` or using `assign()`.

`<<-` comes with the following behaviour: It goes through all environments until it finds the variable in question and replaces it with the current value or creates it anew if it's not found. (Everything run in R 4.0.)

```
> global_x <- 1
> global_x
[1] 1
> meanFUN <- function(x) {
+  global_x <<- mean(x, na.rm = TRUE)
+ }
> meanFUN(c(1:5))
> global_x
[1] 3
```

This will create a new variable called `global_x` in the global environment from which it can be accessed and `global_x` will be changed each time the function is run. However, if `global_x` is found in an environment before the global environment, it will be changed there. If we have a function inside a function with the same variable name across different environments and use the `<<-` operator, `<<-` will only change the first occurrence of the variable in question, so to speak in the overarching local environment of the outer function and not in the global environment.

```
> global_x <- 1
> global_x
[1] 1
> meanFUN <- function(x) {
+   global_x <- 100
+ 
+   insideFunction <- function(x) {
+     local_x <- mean(x, na.rm = TRUE)
+     assign("global_x", local_x, envir = .GlobalEnv)
+     print(global_x)
+   }
+ 
+   print(global_x)
+   insideFunction(x)
+ }
> meanFUN(c(1:5))
[1] 100
[1] 3
> global_x
[1] 1
> 
```

To avoid mistakes while using `<<-`, it's better to use `assign()` which gives precise control over what is assigned where.

```
> global_x <- 1
> global_x
[1] 1
> meanFUN <- function(x) {
+    global_x <- 100
+   
+     insideFunction <- function(x) {
+       global_x <- mean(x, na.rm = TRUE)
+       assign("global_x", global_x, envir = .GlobalEnv)
+       print(global_x)
+     }
+   
+     print(global_x)
+     insideFunction(x)
+   }
> meanFUN(c(1:5))
[1] 100
[1] 3
> global_x
[1] 3
```

On Stack Overflow is a similar question where it's explained in even greater detail: https://stackoverflow.com/a/10904810/3884967.