Install Question Update

[Atomic Marshmallow]

Hello,

Sorry if this is a banal question, but I am wondering whether the new update can be used as a clean install. I will be reformatting my drive shortly, so will I have to install my original disc first or can I use the update I downloaded without installing previous versions?

Also, while I am on the subject. When new updates become available, what is the best way to proceed with them? Is it just double click and install over the existing program or can I delete the old version and install the new?

Thanks in advance for your response(s)