I got a tableview cell inside that i have image view , label and a ui button , i need to change the button title text and background colour on click . But when i try to do
Set the title and the background color properties of the button in tableview's didSelectRowAtIndexPath delegate
cell.button.setTitle("title", for: .normal)
cell.button.backgroundColor = .darkGray
You don't need to assign button.tag at all, Here is the way:
First of all add target to your button. Like:
cell.button.addTarget(self, action:#selector(handleRegister(_:)), for: .touchUpInside). 
Then:
//This function will be called for each button click. 
func handleRegister(sender: UIButton) {
let buttonPosition = sender.convert(CGPoint.zero, to: YOUR_TABLE_VIEW)
let indexPath: IndexPath? = YOUR_TABLE_VIEW.indexPathForRow(at: buttonPosition)
let cell = YOUR_TABLE_VIEW.cellForRow(at: indexPath as IndexPath)
//Now change the text and background colour
cell.button.setTitle("Button Title",for: .normal)
cell.button.backgroundColor = UIColor.blueColor()
   //...
}
To keep the track of your changed button background and title you will need an array as @Tofaani Kaanudo's answer has suggested.
First of all you should take one Array for manage clicked/changed UIButton for reusableCell otherwise if you scroll your tableView Up-Down then It will be remove effect 
I'm giving my logic.
1) Take one mutable Array name is temArray (Size equal to your tableView's Row) and it has 0 value for each index. you can do it by easy repeat value 0.
2) in you cellForRowAtIndexPath datasource method check 
if temArray[indexPath.row] == 0 {
   /// Write code for default UIButton - That has normal behavior means not changed title and no set background color
}
else {
  /// Write code for What you want to keep UIButton title and background color
}
cell.yourButtonObject.tag = indexPath.row
cell.yourButtonObject.addTarget(self, action:#selector(handleButtonClicked(_:)), for: .touchUpInside). 
3) And add handleButtonClicked method and change temArray value 
func handleButtonClicked(_ sender: UIButton) {
        let myIndexPath = NSIndexPath(row: sender.tag, section: 0)
        let cell = tblViewPref.cellForRow(at: myIndexPath as IndexPath)
        if temArray[sender.tag] == 0 {
        /// You can access your button by
        // cell.myButton..... change here text and background color
        // And change "temArray"
        temArray[sender.tag] = 1
       }
       else {
           /// You can access your button by
          // cell.myButton..... change here text and background color
          // And change "temArray"
         // SET DEFULT BUTTON TITLE AND BACKGROUND COLOR
         temArray[sender.tag] = 0
       }
    }
So when you scroll your table Up-down temArray will be managed by it's value at indexPath in cellForRowAtIndexPath Datasource method.