Good question. In my opinion there actually isn´t much of a difference. "Ändern" is used a lot more so in doubt just use that word. "umändern" I know in very few contexts, if for example you would like to adjust the size of a suit because its a bit to long, you might say "den Anzug umändern". Anyways, I wouldn't worry too much about these words while learning German Hope I could help Have a great day :-)