Can upset be used as a substitute for angry?

No it can't. When a person is upset, he or she is not necessarily angry. Some event has just made them feel low. Anger is different. If you're angry, reactions will ensue. However, when you're upset, you feel too lackadaisical to do anything.

Upset indicates depression. 

Angry is high level of emotion. 

I think they are opposite. 

To your question. Yes. It can be used in American English.

But they don't really mean the same thing. Upset, just ,means this is disturbing. 

Not really. To be upset is to feel sad whereas anger is an emotion  generated by the actions of others which are perceived as bad.

Yes, although someone can be upset, but not angry. The meaning, and connotation of angry, depend upon how it is used and the context or circumstances causing the upset. Other ways of saying one is upset are many, but two of the most common are annoyed and irritated.

In some situations, yes; in others, no. Being 'angry' is generally a stronger, fiercer emotion than being 'upset'.

And a person who is 'upset' may not be angry at all. Maybe they saw a cat run over by a car on their way to work. They would be upset, but not necessarily angry.

Not really. You can be upset(death), but not angry. Angry is how you feel when someone lies about you, for example.

It’s a broader term, where angry defines the upset. 

No.

Angry is 

irate

annoyed

cross

vexed

irritated

exasperated

indignant

Upset is

distressed

troubled

perturbed

disturbed

unsettled

disconcerted

worried

bothered

inconvenienced

Yes it means pretty much the same thing.