try this below code
if let button = sender as? UIButton {
        if button.isSelected {
            // set selected
            button.isSelected = true
        } else {
            // set deselected
            button.isSelected = false
        }
    }
For shorthand, try this
if let button = sender as? UIButton {
     button.isSelected = !button.isSelected
}
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solved selected and deselected checkbox in Swift