Typically you say "go back" when talking about a place that is NOT your current location, and you say "come back" when you ARE located at the place/destination.
An example will make it clearer:
I am from the United States, and I am currently living in Brazil. If I plan to move back to the U.S., then I would say "I'm going back to the U.S. next month." I say "going back" because I am currently NOT in the U.S.
But my parents, who are in the U.S., would say "Our daughter is coming back to the U.S. next month." They say "coming back" because they ARE currently in the U.S.
You go back to a place where you have traveled to before (but you don't live there). I've been to Germany before - and I'm going back to Germany for another visit next year.
You get back to a place where you typically stay / live. My husband and I just took a 3-day trip to another state, and we got back to our city this afternoon.
Here's another example - let's say I went to the store and bought some milk, but forgot to buy eggs. This means I need to go to the store again. I'd say "I'm going back to the store - I'll get back (home) in about 15 minutes."
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