You can see sample of your data in two ways-
1) When you create a new connection -
When you create a new data source connection as explained in here: How to Create a new Datasource Connection , you can see a sample of data on the preview screen and perform various type of data wrangling operations on it.
2) Preview Screen for already created datasources-
There is a list of already created datasource connections in the system on connect page.
You can reopen preview screen and see sample data of already created datasources by clicking on "Edit Dataset" it from the connect page.
As shown in below image if you click on SmartPhoneSalesData it will give you its data preview.
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