This is not something that you can just do, like changing the official bird or how often the Judge is elected. The government type can only be changed when the realm is in Anarchy.
There are two ways to get the realm into Anarchy: Have a successful rebellion (with the rebels actually winning by force, not the ruler stepping down), or have an unsuccessful ruler election.
In the former case, the rebel leader then becomes the ruler and can decide the new government type. In the latter case, a special election is held in the capital of the realm for the new ruler, who can then, once again, decide the new government type.