To edit an existing template, you need to open Importacular, select your data source (you must use the data source that matches your template and if you are working with a File, you must have the same column headers that you did when you built the template).
Once you have selected your data, you'll come to the screen that looks like this:
Use the dropdown in the middle to select the template that you want to edit and then click on the pencil icon at the far right of that row.