If this is too basic I apologise as I'm not sure which bits you don't know.
First you need to query the database for the record you want to change
eg. $query=mysql_query("SELECT phoneno from employee where empid-'$id' ");
$row=mysql_fetch__array($query)
then display the results in a form (storing the id in a hidden field so it too gets sent
<form action="update.php" method="post">
<input type="hidden" name="id" value="$id">
<input type="text" name="phone" value="<?=$row["phoneno"]?>">
<input type="submit" value="Update">
</form>
In the update.php you can pick up the editted phone number and id with
$phoneno = $POST["phone"];
$id = $POST["id"];
All you need now is an update query
mysql_query("UPDATE employee SET phoneno='$phoneno' WHERE empid='$id'"
Having done the update you might then want to redirect to another page with
header("location: another.php");
hth