How to improve the taste of sour coffee beans? I was given a few packs of grain coffee. The smell was pretty nice and it was written that it's 100% arabica. That encouraged me to grind some and brew it in moka pot.

The result I got was not horrible, but way to sour for me. I tried one bean and indeed it was little sour, compared to Starbucks Espresso Roast, which is not sour at all and has great almost black color.

I am sure this will not be my coffee of choice, but it would be nice to upgrade it somehow (before I throw it away), though I have no idea how to do it. I thought about roasting it in the oven (210-225 Celsius; 410-437 Fahrenheit) for 10 minutes.

Do you think it might be a good idea?
I don't think I would re-roast them.   I think you would be better off trying a different grind, or preparing the coffee with a different method.   Find ways to work with the acidity, basically.   

A coarse grind and a french-press might work a bit better with the brightly acidic beans.