I need some guidance on Read-only mode SaaS deployment

The feature is covered in the documentation.

How can I disable it after the update?

You don't, you only enable read-only mode on the clone of the live application, not the live application itself.

Do I need to upload a new zip file again which the read-only mode is disable?

No, the updates that are being deployed replace the live application while traffic is routed to the read-only clone. Once the update is complete you can route traffic back to the live application and delete the clone.

I realize your questions are more about the deployment side of things, but read-only mode is implemented in the Kentico Community Portal if you want to see how the visitor experience is updated to handle read-only mode.