How do I change from "Unknown" to show who is calling me? I'm using Samsung Galaxy Mini S3. When one of my contacts calls me I see "unknown" on the screen. 

How do I change it to show who is calling me? It used to be something that has changed.
Normally, it is the person calling you that chooses to call as unknown. Even if it is someone you know and that person is in your contacts, if they have their caller ID turned off, they will appear as unknown on your device.

However, if this happens for every single one of your contacts calling you, I doubt that it is them having their caller ID turned off. If their number appears, but not their name, then caller ID is turned on and it is your phone that is causing problems. Check if your accounts are synced and your contacts have their names set to the correct phone numbers.

If no number appears, no caller ID is coming through. I've heard about this problem before, and probably the problem is not your phone, but your service provider. Try contacting customer support and explain your problem to them. They should be able to help you further by enabling caller ID from their side.