Wonderful question, and my answer will depend on your culture. If by "modest" you mean "not immodest" then good. (Beware that excessive modesty can undermine your career.)
In American culture the following physical behaviors can signal confidence, according to The Emotion Thesaurus: A Writer's Guide To Character Expression (a resource I use when teaching Emotional Intelligence to engineers) - this is an abbreviated list, partly paraphrased:
- Strong posture (shoulders back, chest out, chin high)
- Walking with wide steps
- Strong hygiene and personal grooming
- Direct eye contact
- An easy smile
- Appearing relaxed
- A strong handshake
Obviously you will want to behave in ways that convey confidence in your culture.
While the other advice here is excellent -- saying "thank you" is always good -- I would also suggest you consider where you want to go in your career. You seem proud of advancing quickly. You might consider thanking your boss and your boss's boss, AND adding: "I've done well because I have great teammates and I love growing and pushing myself to higher levels, and you've really supported me by giving me stretch assignments. I'm hoping you'll continue." (Or whatever feels truthful to you.)
When you connect your success with the fact that they've given you increasing responsibility, you're inviting them to continue to increase their trust in you.
And THAT shows confidence. ;-)