Press Edit on their card and change it there. If it's a parent shown with the wrong sex, there's also a choice for that in the parents' fix list.
Where to find it
Person page → Edit, far right
Only people with the "Edit" tick.
On a phone: if the card's buttons are tucked away, press ☰ Menu first.
Still stuck? Talk to a real person: send us a message, or email support@mywholefamilytree.com