How to keep the current tab in .aspx page? After page submitting

 </form>
  </div>
            
 
  <div class="tab" id="tab2">   <h5>Educational Qualification </h5> <br />
     <form method="POST" action="tab2">
<div class="row">
    <div class="mb-3"><asp:dropdownlist runat="server"  class="form-control">
    <asp:ListItem Value=""  selected hidden>Highest Education</asp:ListItem>
    <asp:ListItem Value="R1">Grade 8</asp:ListItem>
    <asp:ListItem Value="R2">O/L</asp:ListItem>
    <asp:ListItem Value="R3">A/L</asp:ListItem>
    <asp:ListItem Value="R4">Degree</asp:ListItem>
     <asp:ListItem Value="R2">Doctor</asp:ListItem>
    <asp:ListItem Value="R3">PHD</asp:ListItem>
    <asp:ListItem Value="R4">Not Mentioned</asp:ListItem>
    </asp:dropdownlist> </div>
    </div>
      <br />
        <h5>Professional Qualification</h5>
      <br />
<div class="row">
    <div class="col">
        <div class="mb-3"><asp:textbox runat="server" type="text" class="form-control"  placeholder="Institute" ID="txtInstitue"></asp:textbox></div>

    </div>
    <div class="col">
        <div class="mb-3"><asp:textbox runat="server" type="text" class="form-control" ID="txtQualification"  placeholder="Qualification"></asp:textbox></div>

    </div>
    <div class="col">
        <div class="mb-3"><asp:textbox runat="server" type="text" class="form-control" ID="txtDuration"  placeholder="Duration"></asp:textbox></div>

    </div>
     <div class="col">
         <asp:button runat="server" text="Add"  class="btn btn-primary" ID="btnAdd" OnClick="btnAdd_Click" />
    </div>
</div>
      <br />
      <asp:gridview runat="server" ID="grdProfessionalDetails"></asp:gridview>

     </form>
  </div>

        
  <div class="tab">Birthday:
    <p><input placeholder="dd" oninput="this.className = ''" name="dd"></p>
    <p><input placeholder="mm" oninput="this.className = ''" name="nn"></p>
    <p><input placeholder="yyyy" oninput="this.className = ''" name="yyyy"></p>
  </div>
  <div class="tab">Login Info:
    <p><input placeholder="Username..." oninput="this.className = ''" name="uname"></p>
    <p><input placeholder="Password..." oninput="this.className = ''" name="pword" type="password"></p>
  </div>
  <div style="overflow:auto;">
    <div style="float:right;">
      <button type="button" id="prevBtn" onclick="nextPrev(-1)" class="btn btn-secondary">Previous</button>
      <button type="button" id="nextBtn" onclick="nextPrev(1)" class="btn btn-primary">Next</button>
    </div>
  </div>
  <!-- Circles which indicates the steps of the form: -->
  <div style="text-align:center;margin-top:40px;">
    
    
    
    
  </div></div>
    <div style="text-align:center;">
    <p class="mt-5 mb-3 text-muted">©2021 SLFEA</p>
       <p class="mb-3 text-muted" style="font-size:small;">Designed by Dockyard Total Solutions Pvt Ltd</p></div>

    <script>
var currentTab = 0; // Current tab is set to be the first tab (0)
showTab(currentTab); // Display the current tab

function showTab(n) {
  // This function will display the specified tab of the form...
  var x = document.getElementsByClassName("tab");
  x[n].style.display = "block";
  //... and fix the Previous/Next buttons:
  if (n == 0) {
    document.getElementById("prevBtn").style.display = "none";
  } else {
    document.getElementById("prevBtn").style.display = "inline";
  }
  if (n == (x.length - 1)) {
    document.getElementById("nextBtn").innerHTML = "Submit";
  } else {
    document.getElementById("nextBtn").innerHTML = "Next";
  }
  //... and run a function that will display the correct step indicator:
  fixStepIndicator(n)
}

function nextPrev(n) {
  // This function will figure out which tab to display
  var x = document.getElementsByClassName("tab");
  // Exit the function if any field in the current tab is invalid:
  if (n == 1 && !validateForm()) return false;
  // Hide the current tab:
  x[currentTab].style.display = "none";
  // Increase or decrease the current tab by 1:
  currentTab = currentTab + n;
  // if you have reached the end of the form...
  if (currentTab >= x.length) {
    // ... the form gets submitted:
    document.getElementById("regForm").submit();
    return false;
  }
  // Otherwise, display the correct tab:
  showTab(currentTab);
}

function validateForm() {
  // This function deals with validation of the form fields
  var x, y, i, valid = true;
  x = document.getElementsByClassName("tab");
  y = x[currentTab].getElementsByTagName("input");
  // A loop that checks every input field in the current tab:
  for (i = 0; i < y.length; i++) {
    // If a field is empty...
    if (y[i].value == "") {
      // add an "invalid" class to the field:
      y[i].className += " invalid";
      // and set the current valid status to false
      valid = false;
    }
  }
  // If the valid status is true, mark the step as finished and valid:
  if (valid) {
    document.getElementsByClassName("step")[currentTab].className += " finish";
  }
  return valid; // return the valid status
}

function fixStepIndicator(n) {
  // This function removes the "active" class of all steps...
  var i, x = document.getElementsByClassName("step");
  for (i = 0; i < x.length; i++) {
    x[i].className = x[i].className.replace(" active", "");
  }
  //... and adds the "active" class on the current step:
  x[n].className += " active";
}
</script>