Change float value with Button Onclick

I have this timer script to let an object disappear.
By giving sec a value I can adjust the time.
But now I want to access and change this value from Button > OnClick so I started function SecVideo.
How do I let this function make adjusting the float sec possible with a button click event?

using UnityEngine;
using System.Collections;

public class timer : MonoBehaviour {

	public float sec;
	public void SecVideo(float sec) 
	void Start()
	{	StartCoroutine(LateCall());
	IEnumerator LateCall()
		yield return new WaitForSeconds(sec);
		//Do Function here...
	// Update is called once per frame
	void Update () {

Perhaps like:

public void SecVideo(float sec)
    this.sec = sec;

and in unity this parameter sec should be appear like specified in this image: