Hi Debbie,
Yes, that’s all you have to do for finishing the service upgrade.
After you finish it, all the contents and sub-sites can be found in your upgraded SharePoint site.
However, the User Interface (UI) has been changed and you need to take some time to get used to it.
As you mentioned the sub-site in your first post. It can be found in the “Site content” as the screenshot shows below:
I am very glad to assist you if you have any questions about using the new UI.
Thanks,
Wilfred