Where is JFET footprint?

Modifying schematic symbols ( or creating new ones) is done with the schematic symbol editor. It is part of KiCad, but a separate program. You start it with the icon next to the EEchema icon in the KiCad project manager:

The user interface is very similar to that of Eeschema itself, but to be able to work with it, you also have to learn some library management.

In an earlier post here I already posted a link to a tutorial for the schematic symbol editor.

Here is another one for the library management