After uploading your data, you will be directed to the Data Source where you can view your data. (See Sales Commission example below.)
You can edit your data in this section. It is recommended that you make your data as clean as possible in Tableau. This means making sure value labels are used consistently, removing leading zeros or spaces, and missing values and handled appropriately for your type of data.
To get more information or to edit the data field, hover above the data field. You can view a drop-down arrow and you will see different options available to you.
If you want to add more files, go to the top left corner of your screen. Once there, click the ADD button and upload your file. Once you upload another file, you will be able to see both files on the left sidebar.
Tableau automatically assigns data types to your data fields. If for some reason Tableau fails to do so, you can also edit it yourself.
To edit the data type, you have to go to the sheet tab on the bottom next to data source. (It will by named by default Sheet1). Once you are in the sheet, on the left corner you can see the data fields. Click the icon in front of the data field and change it into the type you want it to be.