How to access the content of a custom cell in swift using button tag?

OPTION 1. Handling it with delegation

The right way of handling events fired from your cell's subviews is to use delegation.

So you can follow the steps:

1. Above your class definition write a protocol with a single instance method inside your custom cell:

protocol CustomCellDelegate {
    func cellButtonTapped(cell: CustomCell)
} 

2. Inside your class definition declare a delegate variable and call the protocol method on the delegate:

var delegate: CustomCellDelegate?

@IBAction func buttonTapped(sender: AnyObject) {
    delegate?.cellButtonTapped(self)
}

3. Conform to the CustomCellDelegate in the class where your table view is:

 class ViewController: CustomCellDelegate

4. Set your cell's delegate

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as CustomCell
    cell.delegate = self

    return cell
}

5. Implement the required method in your view controller class.

EDIT: First define an empty array and then modify it like this:

private var selectedItems = [String]()

func cellButtonTapped(cell: CustomCell) {
    let indexPath = self.tableView.indexPathForRowAtPoint(cell.center)!
    let selectedItem = items[indexPath.row]

    if let selectedItemIndex = find(selectedItems, selectedItem) {
        selectedItems.removeAtIndex(selectedItemIndex)
    } else {
        selectedItems.append(selectedItem)
    }
}

where items is an array defined in my view controller:

private let items = ["Dog", "Cat", "Elephant", "Fox", "Ant", "Dolphin", "Donkey", "Horse", "Frog", "Cow", "Goose", "Turtle", "Sheep"] 

OPTION 2. Handling it using closures

I've decided to come back and show you another way of handling these type of situations. Using a closure in this case will result in less code and you'll achieve your goal.

1. Declare a closure variable inside your cell class:

var tapped: ((CustomCell) -> Void)?

2. Invoke the closure inside your button handler.

@IBAction func buttonTapped(sender: AnyObject) {
    tapped?(self)
}

3. In tableView(_:cellForRowAtIndexPath:) in the containing view controller class :

let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath) as! CustomCell       
cell.tapped = { [unowned self] (selectedCell) -> Void in
    let path = tableView.indexPathForRowAtPoint(selectedCell.center)!
    let selectedItem = self.items[path.row]

    println("the selected item is \(selectedItem)")
}

Since you have 1 section in the table view you can get the cell object as below.

let indexPath = NSIndexPath(forRow: tag, inSection: 0)
let cell = tableView.cellForRowAtIndexPath(indexPath) as! CustomCell!

where tag you will get from button tag.

Swift 3 I just get solution for access cell in @IBAction function using superview of button tag.

let cell = sender.superview?.superview as! ProductCell
var intQty = Int(cell.txtQty.text!);
intQty = intQty! + 1
let  strQty = String(describing: intQty!);
cell.txtQty.text = strQty