Change in company culture: How to do it?

Much has been written about company culture. You know its importance for the perceptions of customers and other people outside the company, as well as how it is perceived by employees themselves. It defines the identity of the company, connects the employees and motivates them to a better performance. A company culture that unifies, is clearly defined and embraced by everyone is important in all aspects. But if you want to alter anything in it, how do you achieve this? In other words, how can you handle company culture and make changes to it?