FOR loops count up by one by default. They aren't smart enough to figure out on their own to count backwards, so you have to tell them with the STEP keyword:
FOR I=C-1 TO 0 STEP -1 PRINT ARR$[I] NEXTWith STEP, you're just telling the loop to count by a different number. Counting up by -1 is the exact same as counting down by 1, mathematically. It could be any number: 12, -5, 23.4748383, whatever.