My Sanctuary Commander is a 2 star, +2. How do I replace with a 3-star?

Your sanctuary commander doesn't have Stars. You can tap the avatar in the top left to see your commander info screen which shows gear, chips and other stats, plus check the talents and other info at the bottom. Talents are unlocked by earning Commander Exp.

You may be using an avatar image that is one of the heroes in the game, but this is just the image you have and will appear as in chat etc. The level of that hero that you have does not affect your commander stats.

You can change the image you have in the Commander Info screen by tapping the avatar near the bottom left.