You have to actually move the location of the point on the map or edit the coordinates.
Pinned locations aren’t available as an option when editing observations on the website. There is a feature request for this: https://forum.inaturalist.org/t/select-a-pinned-location-on-the-edit-page-for-already-uploaded-observations/3626/
If you use the android app, it should be possible to access pinned locations when editing.