It's not easy to edit footprint directly from your schematic, at least I don't know any method to do it.
Anyway, there is another method: first you need to find your part in library, then click on footprint on the right. It should open in another tab, edit what you need, then save it under new name (copy name to clipboard for later).
Next, go to main menu, select Tools > Footprint Manager, then select your part on the left, on the right in search window paste your edited footprint name, select it on list below (sometimes you get more than one result if your new footprint name is not unique enough) then click Update button below.
Now your part in schematic should have new footprint. All that's left is to save schematic and update pcb.