You can earn Experience Points by upgrading your buildings or troops in the Armory, or with new construction.
Tap on the blue circle in the upper left corner of your screen to see how many Experience Points you have, and how many you need to reach the next level.
You need a certain number of XP points to be able to upgrade your Headquarter to the next level. This is because we don’t want players to rush their HQ to a high level and leave all their other buildings and troops at a low level as it would negatively influence your gameplay experience.