Use a selection tool based on edges, not just color alone. If you try an automatic color-based selection, lower or adjust the tolerance so it recognizes the edge of the dress, and enable a setting like contiguous so it only selects connected background areas instead of every white area in the image.

If the background white is uneven, try using a magic wand or similar selection tool to trace/select the background around the dress and see whether it snaps cleanly to the subject edges.

If the dress and background aren’t clearly separated enough for automatic tools, you’ll likely need to do it manually. A common method is to duplicate the image to a new layer, place a bright solid color layer underneath, then erase or mask the background while checking the edges against that bright color. This makes it easier to see leftover background around the dress.

In short: start with tolerance + contiguous selection, then refine manually if needed.