How to Create a Method with non constant variables in swift

Question

I am working on a project in swift and need to create a new method, where some of the variables initialized with it are not the default constant, in this case specifically in the method itself, I need to modify the variables it is initialized with. After searching for a bit of time, I was wondering how I would do such a thing. The method and its corresponding error are listed below.

 func glow(body: SKSpriteNode, NewColor: UIColor, OGColor: UIColor, time: Int, check: Bool, count: Int) {
        if check {
            body.color = NewColor
            count = 0
        }
        if count >= time {
            count = time
        }
      
        let RedChangePerFrame = (CGFloat(-1 * (CGFloat(NewColor.rgb()!.red) - CGFloat(OGColor.rgb()!.red)) / 255) / CGFloat(time))
        let red = (CGFloat(NewColor.rgb()!.red) / 255) + (RedChangePerFrame * CGFloat(PlayerFadeCount))
        
        let GreenChangePerFrame = (CGFloat(-1 * (CGFloat(NewColor.rgb()!.green) - CGFloat(OGColor.rgb()!.green)) / 255) / CGFloat(time))
        let green = (CGFloat(NewColor.rgb()!.green) / 255) + (GreenChangePerFrame * CGFloat(PlayerFadeCount))
        
        let BlueChangePerFrame = (CGFloat(-1 * (CGFloat(NewColor.rgb()!.blue) - CGFloat(OGColor.rgb()!.blue)) / 255) / CGFloat(time))
        let blue = (CGFloat(NewColor.rgb()!.blue) / 255) + (BlueChangePerFrame * CGFloat(PlayerFadeCount))
        
        body.color = UIColor(displayP3Red: red, green: green, blue: blue, alpha: 1)
        PlayerFadeCount += 1
        
    }

Cannot assign to value: 'count' is a 'let' constant 

For additional information, when I run the method I set "count" to a variable called PlayerFadeCount, which is always set to an Int. Any and all help is appreciated!