Select the Dashboard you wish to set. Click on the "Customize" button then select "Set as Default"

As an admin, you can set the default for other users. If you want to set it as the default for just yourself, select the "For Me" button. Otherwise, to set it as the default for other org levels, select the "For Others" button.

Select the org levels that you want this dashboard to be the default for. All users within the selected org levels will see this dashboard when they first log into the system. Once you have selected the org levels, click "Set Default Dashboard" to finish.