How to create a form that will ask for a record you want to update in php.

Here are some general guidelines. My apologies if I go over stuff you already know.

The first question is whether you want the user of the page to select the record to edit. If so, then you need to provide some way for users to do that.

The next step is to query the database and bring back the record you want to update (or the record the user selected to update).
For this you would write a query, and you would submit it to the web server to pass on to the database server using mysqli or PDO:

<?php

$sql = "SELECT * FROM table_name WHERE recordID = nnn";
---mysqli or PDO code here---
$results = . . .
?>

This PHP can be at the top of the html page with the form, or it can be a separate page. If it's at the top of the same page, make sure you have some code to keep it from executing until the record has been selected. This is usually done in a PHP if statement.

The third step, after you get the record from the database, is to display it in the form fields.
The results of the query will come back as a recordset, which will be stored in a variable, say $results. Then, in each of the form's elements you would echo the appropriate returned value. E.g., if you have a textbox for firstName, you would do this:

<input type="text" name="firstName" id="firstName" value="<?php echo $results['firstName']; ?> />

It's a little bit trickier to echo for check boxes, radio buttons and selects, but there are plenty of instructions on the web on how to do that. For each type you would use $results[. . .] with the appropriate field name inside the brackets in one way or other.

If you know Javascript and AJAX, you can do all of this without a full page refresh. Which provides a nicer user experience. They select a record to update, and the form fields get filled!

Let me know if you need more detail on any of the steps.