Two things need to happen to make a dashboard your default:
Performed by an Admin user:
In the top left, use the dashboard drop-down and select Manage Dashboards.
Search for the required dashboard and open the record.
Select the **Default Dashboard** checkbox.
Click Save.
Set on a user-by-user basis:
In the top left, use the dashboard drop-down and select Manage Dashboards.
Search for the required dashboard, but do not open the record.
Next to the name, click the star icon. This will mark it as your favourite.