How to add text on end of each line in text file?

File.WriteAllLines(filePath, File.ReadAllLines(filePath).Select(line=>line+"OK"));

string textToAppend = "ok";
List<string> lines = new List<string>();
foreach (var line in File.ReadAllLines(filePath))
{
    lines.Add(line + textToAppend);
}

File.WriteAllLines(filePath, lines.ToArray());

this is entire code, i want to move file (in button btnSave) and if file exist to overwrite that file, how to do that?

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Threading;



namespace ToEditTxt
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            
            // sakrivanje dugmeta -obradi- i dugmeta -snimi- i promena boje
            btnEdit.Enabled = false;
            btnEdit.BackColor = Color.White;
            btnSave.Enabled = false;
            btnSave.BackColor = Color.White;
        }

        private void btnLoad_Click(object sender, EventArgs e)
        {
            //menjanje boje dugmeta i sakrivanje dugmeta
            btnLoad.Enabled = false;
            btnLoad.BackColor = Color.White;

            // omogućavanje dugmeta  -obradi- i promena boje
            btnEdit.Enabled = true;
            btnEdit.BackColor = System.Drawing.ColorTranslator.FromHtml("#d3d3d3");
            



            

            //učitavanje fajla
            openFileDialog1.ShowDialog();
            label1.Text = openFileDialog1.FileName;
            label2.Text = System.IO.Path.GetFileName(openFileDialog1.FileName);
            label3.Text = System.IO.Path.GetFileNameWithoutExtension(openFileDialog1.FileName);

            label4.Visible = true;

            
        }





        private void btnEdit_Click(object sender, EventArgs e)
        {
            //menjanje boje dugmeta i sakrivanje dugmeta
            btnEdit.Enabled = false;
            btnEdit.BackColor = Color.White;


            //dodavanje 1 na kraju svakog reda u texualnom fajlu
            string textToAppend = "1";
            List<string> lines = new List<string>();
            foreach (var line in File.ReadAllLines(label1.Text))
            {
                lines.Add(line + textToAppend);
            }

            File.WriteAllLines(label1.Text, lines.ToArray());

            // omogućavanje dugmeta  -snimi- i promena boje
            btnSave.Enabled = true;
            btnSave.BackColor = System.Drawing.ColorTranslator.FromHtml("#d3d3d3");


           
        }

        private void btnSave_Click(object sender, EventArgs e)
        {
            try
            {
                //snimanje i preacivanje fajla u drugi direktorijum
                System.IO.FileInfo f = new System.IO.FileInfo(label1.Text);
                f.MoveTo(@"c:\q\" + label3.Text + ".rpt");



                if (File.Exists(@"c:\q\"))
                {
                    File.Delete(@"c:\q\");
                    File.Move(label3.Text + ".rpt", @"c:\q\");
                }
                File.Move(label3.Text + ".rpt", @"c:\q\");
            }

            catch (IOException)
            {



                //resetovanje labela na početne vrednosti
                label1.ResetText();
                label2.ResetText();
                label3.ResetText();
                this.Refresh();
                this.ResetText();
                this.Update();
                // vraćanje grafičkog okruženja na početne vrednosti
                btnLoad.Enabled = true;
                btnEdit.Enabled = true;
                btnLoad.BackColor = System.Drawing.ColorTranslator.FromHtml("#d3d3d3");
                btnEdit.BackColor = System.Drawing.ColorTranslator.FromHtml("#d3d3d3");

                label5.Visible = true;

            }
        }

        private void exitToolStripMenuItem_Click(object sender, EventArgs e)
        {
            this.Close();
        }
    }
}
        
    

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