Reorder stems within a category by dragging, delete and re-upload to replace a track, and keep in mind tracks are project-specific.
Track categories, like vocals, drums, and instruments, have a fixed order. Within a category, though, you can drag and drop individual stems to reorder them however you like.
Delete the incorrect track, then upload the correct file in its place. There's no direct "replace" step. Removing and re-uploading is the way to swap a file.
No. Tracks are specific to the project they were uploaded to. To use the same file in a different project, upload it again there.