Once saved, it is not possible to edit a template. To make changes, clone the template and change the clone.
To modify a template, repeat the steps 1-6 in cloning a template. You will see the template open on Template Creating screen. Here you can add/remove/edit fields as when you create a new template.
To operate on Displayed fields, click Displayed Fields. You will see all the displayed fields. To remove one, highlight it and click Actions > Remove Selected Field. You may move a field up or down the list, which will affect the sorting of the result records. You may also change the column label or the transform of the field by using respective functions on the Actions list.
You may add fields. See Chapter 33, Creating Templates for details on operating on fields.
You can not change the starting source table, but have to follow the links to tables in the Sources pane. Changing the starting table will remove all existing fields, meaning building a new template from scratch.