How do I upgrade my planes?

Follow:

1) Click Air force command center (it's a building in your base)

2) Click the "Air force" tab

3) Click the plane you want to upgrade

4) You'll be directed to a page with 3 tabs located on the bottom of the page. Click on "Air Force Remodelling"

5) You'll be directed to another page with 2 tabs located in the center of the page: a) Upgrade and b) Promote. Click the latter to upgrade your plane. Check for the requirements obviously before you upgrade.