When editing a box, it's important to determine whether you want (a) to edit the original box or (b) to provide a version of the box that's specific to a given space.

If you want the former, ensure you're not in a space before editing. You can tell by the presence or absence of a space prefix in the path. For example, if the path shows http://example.com/global/something, the "global" is the prefix for the global space, indicating you're in a space. Before editing the box, leave the space. You can do so by going to the same path you're on, minus the space prefix. For example, you could go to http://example.com/something. Or you could start at http://example.com/admin and navigate to the page for editing boxes.

If you want the latter, ensure you're in the given space before editing.

If your space is part of a preset, you might then want to add the box override to the preset. When you've edited the box, go to the space's home page, click Features and then Overrides, and save the box override you just created to the space's preset.