mypage.xaml

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
    x:Class="MyFirstOouiApp.Pages.MyPage">
	<ContentPage.Content>
        <StackLayout>
            <Label Text="Hello World" FontSize="Large" 
                   HorizontalOptions="Center" HorizontalTextAlignment="Center"/>
            <Button Text="Click Me!!!" Clicked="Handle_Clicked" HeightRequest="50"/>
        </StackLayout>
	</ContentPage.Content>
</ContentPage>

mypage.xaml.cs

using System;
using System.Collections.Generic;

using Xamarin.Forms;

namespace MyFirstOouiApp.Pages
{
    public partial class MyPage : ContentPage
    {
        public MyPage()
        {
            InitializeComponent();
        }

        int count = 0;
        void Handle_Clicked(object sender, System.EventArgs e)
        {
            var button = sender as Button;

            if (button == null)
                return;

            count++;
            button.Text = $"You clicked {count} times!";
        }
    }
}