Maybe...

// are comments 

var p := 2; //Position to change
var t := "f"; //change it To
var x := ["a", "b", "c", "d", "e"]; //data
var a := slice(x, 0, p);//data before position
var b := slice(x, p, p + 1);//the position data for clarity
var c := slice(x, p + 1, cnt(x));//data after the position
x := split(concat(a) + ", " + t + ", " + concat(c), ", ");//put it back together and make it an array — watch the spaces with the commas

Agreed

let myArray := ["a", "b", "c"];
myArray := [item(myArray, 0), "f", item(myArray, 2)];
alert(text(myArray))

It's not great, but it does preserve the values you don't want to change.

Hi Elena Rodriguez

How about something like:

let array1 := ["a", "b", "c"];
let changeItem := 2;
for i in array1 do
    if index(array1, i) = changeItem - 1 then
        "f"
    else
        i
    end
end

the result is

a,f,c

You can change the value in line 2 to 1,2, or 3. Or if your array is bigger to any number that makes sense in the array.

We use the handy index() command to get us the space in the index the value occupies. In Ninox all arrays start with 0, so that is why in line 4 we have to subtract one from the changeItem value. I left it this way so it is more human friendly as we think count the values in an array starting with 1.