Next we need to define the increments. We have to use operators for that; using the = sign would set the value of the scroll position to whatever we placed after it, but we don't want to do that. Instead we want to take the current position--no matter what it might happen to be--and, since we're working on the "up" button, lower that position's value by one. So we add a - sign in front of the =, and a 1 after, effectively translating into, "when we click on the up button, the content of the text box named "scrollbox" will scroll up by a single increment from its current position".
I know it seems a little confusing, but after you work with it for a while you get used to it. You can edit the code to add the appropriate variables/conditions in the Expressions text box, so it should look similar to this:
on (release) {
scrollbox.scroll -=1;
}
