You need to invalidate it and recreate it. You can then use an isPaused
bool to keep track of the state if you have the same button to pause and resume the timer:
var isPaused = true
var timer = NSTimer()
@IBAction func pauseResume(sender: AnyObject) {
if isPaused{
timer = NSTimer.scheduledTimerWithTimeInterval(0.1, target: self, selector: Selector("somAction"), userInfo: nil, repeats: true)
isPaused = false
} else {
timer.invalidate()
isPaused = true
}
}