Upgrading your City Official's level 

Through the city's development (construction, researching technologies, training troops, etc.) the City Official will receive experience points (Exp).

Using items, such as an Experience Chest, can greatly increase your Hero's experience and overall level.

Upgrading your normal Hero's Level 

Heroes can earn experience by Plundering and Occupying cities and Wildlands. 

Heroes can also earn experience by gathering resources and treasure at Wildlands.  

Some items can be used to increase a Hero's experience and overall level.