Go to the dataset and click on 'Edit'. Are there too few columns? Click on 'Delimiter' and modify the delimiter, which is the character used to seperate the columns. Are the names of the columns incorrect? Click on 'Column names', on this page you can change the column names. You can also set whether the first line of the file contains the column names or not.
Go to the dataset and click on 'edit'. Below the data example you can find the button 'Add data'. When you click on this button, you can upload a file. Then you can add the data from this file to your dataset. Make sure you check the data carefully, because adding data cannot be reversed.
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