Hi Rami,
Congratulations to your situation - many would be happy to have that choice at the moment!
Don't worry too much about it. It's almost daily business for HR and basically all applicants apply not only at one firm, so chances are pretty high that you might end up with multiple offers (and sometimes you can't arrange all interviews in a certain sequence which allow having all results on the table at one point in time to choose from).
The main idea is: communicate responsibly. Grab your phone and talk to HR. Be straight-forward and tell them that you decided to take another offer with a short explanation why (and focus how the other firm is a better fit to your skills/background) - at the end of the day both of you should be happy if your fit is better with the other firm and you don't take the offer we talk about.
Hope that helps - if so, please be so kind to give it a thumbs-up with the green upvote button below!
Robert