Navigate to Org settings > Members > All Members. Click Set Role.
Click +Add New on the modal.
Select a Standard Role to assign to the Member.
The Role will start today. Confirm if you want to update the member's Role in all Projects or just on new Projects.
The Role will show on the Member with a start date of today.
When default role is updated on the Member, the Member will also be added to the Role in the Role tab.