Solution 1: Recursively

func upsideDownBinaryTree(_ root: TreeNode?) -> TreeNode? {
    if root == nil || root?.left == nil {
        return root
    }
    let res = upsideDownBinaryTree(root?.left)
    root?.left?.left = root?.right
    root?.left?.right = root
    root?.left = nil
    root?.right = nil
    return res
}

Solution 2: Iteratively

func upsideDownBinaryTree(_ root: TreeNode?) -> TreeNode? {
    var current = root
    var pre: TreeNode?
    var next: TreeNode?
    var tmp: TreeNode?
    while current != nil {
        next = current?.left
        current?.left = tmp
        tmp = current?.right
        current?.right = pre
        pre = current
        current = next
    }
    return pre
}