There are two ways to upload an updated Excel spreadsheet.
Under the Library in your Gallery, click on "All Datasets." Once you find the dataset you would like to update select the "Update" option under the Action column. Select and upload your updated spreadsheet. If you've already created a chart with this dataset, your chart will automatically update across the web with this action. If you are building a new chart, the latest data will load into your studio.
If you are currently in the studio, click on the Data menu and select View Data or the View button at the top right of your studio (1). This will prompt your Data Editor to display. When it does click on "Update File" to select your Excel file from your local desktop (2). Once you select the file, the upload will occur immediately. When the data loads into the Data Editor, close the window and continue formatting. If you've already embedded your chart with this dataset, your chart will automatically update across the web with this action.