If you have not already read the Roles and Permissions Overview, I recommend you do that first.
If you want to change the access level someone has (known as their "Role") this is how to do that:
You must have either an Administrator, Board Secretary or Chair role yourself
Click on the People and People List
Click on the context menu at the end of the row -Edit
When the pop up box opens you can:
Tick or un-tick the Board Member box (if this box is ticked the person will be listed as a Board Member on the Agenda/Minutes, be able to vote, and also allow them to view any confidential folders in Governance Documents).
Tick the box next to their new role
Click on Save Changes.
If the person is online at the time, they may need to log out, and then log back in for the change to take effect.
A person cannot have the role of an Executive/Guest and have the Board Member box ticked.