You can go into the profile of the incorrectly entered person on your tree, and find the proper one if you can find an existing tree and request a merge. Then the other profile manager can approve the merge and from there it can be removed from your tree.
If there is no existing profile to merge with, you can create the new person as a profile and do a little bit of research. Just by amending your existing entry once it has different family members (if you can find these through research, properly souce the profile below) and it will automatically disconnect from your tree. You can go to the mother and father's profile and edit their children and erase the incorrect individual that way so it will not be connected.