Relation between report in Power BI Desktop

This depends on what you mean by updating a data set.

Basically, if you are creating a report for your own use only, you can leave it on the desktop.
A Service allows you to share reports with others.

If you mean a change that you want to initiate (a measure, column, table for data has been expanded, permanent visual changes, etc...) then a PBIX file is acceptable.
When you have made changes to your desktop PBIX file, you need to reshare your PBIX file to Service.
The term "update" refers to refreshing the data as a result of a change in the source of the data.
You have added orders to an order table, for example.
Depending on the type of data source, this is a refresh operation that needs to be defined in the service.

Visit the following link for more information:
https://learn.microsoft.com/en-us/power-bi/connect-data/refresh-data

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Yes.
Plese see an attached picture .

The following is taken from the Microsoft document at the following link:
https://learn.microsoft.com/en-us/power-bi/create-reports/service-dashboard-tiles
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