Simple answer, Hover over any existing footprint in Pcbnew, then press [Ctrl + e] to edit it in the “Footprint Editor”.
The Footprint editor Icon is in KiCad’s project manager, between Pcbnew and Gerbview.
If you do it this way, the modified footprint is directly put back on the PCB. If you want to change the library footprints, then you also have to do some:
(KiCad’s default libraries are read-only, so you have to make copies of footprints).