PHP SQL Table Edit row with inline link

echo "<td><a href='edit.php?id=".$row['ID']."'>Edit</a></td><tr>";

In edit.php you have to create form which values will be loaded from database:

<?php 

if (isset($_POST['fieldName']){
  mysqli_query($con, 'UPDATE `TABLE_NAME` SET fieldName='.$_POST['fieldName'].' WHERE ID='.intval($_GET['id']);
  exit;
}

$row = mysqli_fetch_assoc(mysqli_query($con, 'SELECT * FROM `TABLE_NAME` WHERE ID='.intval($_GET['id']))); ?>
<form method="POST">
<input type="text" name="fieldName" value="<?php echo $row['fieldName'] ?>">
<!--    -->
<input type="submit" value="save edit">
</form>

You need improve this pseudocode with data filtering, add more fields, include db connection etc.

I think you need a javascript code to trigger Edit mode.

But first, you need to give your table fields unique IDs while executing the while loop.

$counter=1;
while($row = mysqli_fetch_array($result))
  {
  echo "<tr>";
  echo "<td><div id='ID_$counter'>" . $row['ID'] . "</div></td>";
  echo "<td><div id='TH1_$counter'>" . $row['rowName'] . "</div></td>";
  echo "<td><div id='TH2_$counter'>" . $row['rowName'] . "</div></td>";
...
  echo "<td><button onclick='triggerEdit($counter)'>Edit this row</button></td><tr>";

$counter++;
}

then using javascript create the edit and update functions:

    function triggerEdit(rowNumber) {
            document.getElementById('id_'+rowNumber).innerHTML = "<input type=text id='SOMEIDx'><button onclick='update(SOME_IDx)'>update</button>";
            document.getElementById('TH1_'+rowNumber).innerHTML = "<input type=text id='SOMEIDx2'><button onclick='update(SOME_IDx2)'>update</button>";
// don't forget to assign the IDs
        }

If you want the old values, you can access them using document.getElementById().innerHTML before you change them.

Finally, Construct an update function that sends the data using AJAX, given the ID of the field.