edit all rows of gridview on button click which is outside gridview

It is recommended to do this in javascript. Javascript is far more better performance as the task is carried out at client browser.

However, if you still want to do it at server side, below will give you an idea:
ASP.NET page code:

<asp:gridview id="GridView1" runat="server" autogeneratecolumns="False" xmlns:asp="#unknown">
    <columns>
        <asp:boundfield datafield="name" headertext="Name" />
        <asp:boundfield datafield="tel" headertext="Tel" />
    </columns>

</asp:gridview>

<br />

<asp:button id="Button_Edit" runat="server" onclick="Button_Edit_Click" xmlns:asp="#unknown">
    Text="Edit" /></asp:button>

C# Code behind:

public partial class WebForm1 : System.Web.UI.Page
{
    System.Data.DataTable dt
    {
        get
        {
            return (System.Data.DataTable)ViewState["dt"];
        }
        set
        {
            ViewState["dt"] = value;
        }
    }

    bool isEditMode
    {
        get
        {
            if (ViewState["isEditMode"] == null)
                return false;
            return (bool)ViewState["isEditMode"];
        }
        set
        {
            ViewState["isEditMode"] = value;
        }
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            dt = new System.Data.DataTable();
            dt.Columns.Add("name");
            dt.Columns.Add("tel");
            dt.Rows.Add("Thomas", "1111");
            dt.Rows.Add("Jessie", "2222");
            dt.Rows.Add("Parker", "3333");
            BindData();
            isEditMode = false;
        }

        if (isEditMode)
            AddTextBox();
    }

    void BindData()
    {
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }

    void AddTextBox()
    {
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            for (int j = 0; j < dt.Columns.Count; j++)
            {
                GridView1.Rows[i].Cells[j].Controls.Add(new TextBox());
            }
        }
    }

    void LoadDataIntoTextbox()
    {
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            for (int j = 0; j < dt.Columns.Count; j++)
            {
                ((TextBox)GridView1.Rows[i].Cells[j].Controls[0]).Text = dt.Rows[i][j] + "";
            }
        }
    }

    void SaveData()
    {
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            for (int j = 0; j < dt.Columns.Count; j++)
            {
                dt.Rows[i][j] = ((TextBox)GridView1.Rows[i].Cells[j].Controls[0]).Text;
            }
        }

        BindData();
    }

    protected void Button_Edit_Click(object sender, EventArgs e)
    {
        isEditMode = !isEditMode;
        if (isEditMode)
        {
            Button_Edit.Text = "Save";
            AddTextBox();
            LoadDataIntoTextbox();
        }
        else
        {
            Button_Edit.Text = "Edit";
            SaveData();
        }
    }
}

thanks adriancs...

i understood the logic..i have used the isEditMode in my textbox and label to check and perform hide and show setting the visibility

<asp:templatefield headertext="Name" xmlns:asp="#unknown">
          <itemtemplate>
               <asp:textbox id="txtEditName" visible="<%# IsInEditMode %>" runat="server" text="<%# Bind("Name") %>"></asp:textbox>


               <asp:label id="lblName" visible="<%# !(bool) IsInEditMode %>" runat="server" text="<%# Bind("Name") %>"></asp:label>
           </itemtemplate>
           </asp:templatefield>

and getting the updated value from my textbox like this on click of save button

protected void btnEdit_Click(object sender, EventArgs e)
        {
            IsInEditMode = true;
            BindGrid();
           
        }
 protected void btnSave_Click(object sender, EventArgs e)
        {
            foreach (GridViewRow row in GridView1.Rows)
            {
                string name = ((TextBox)row.FindControl("txtEditName")).Text;
            }
        }