Make a page. Call it personal_1.php. Every single person who successfully logs in gets redirected to this page. But since you have cookied them (or embed their userid in every URL), you know who they are when they arrive at the personal_1.php page.
That page uses the userid to lookup their current details in the database. Then you use those details to populate a form. For example, let's consider NAME, AGE and EMAIL are the details that you are storing about your users. You look up their details in the database, and then you build a form like this:
<form action="personal_2.php">
<input type=hidden name="id" value="<?php print "$id"; ?>">
Name: <input type=text name="name" value="<?php print "$name"; ?>"><br>
Email: <input type=text name="email" value="<?php print "$email"; ?>"><br>
Age: <input type=text name="age" value="<?php print "$age"; ?>"><br>
<input type=submit value="Update">
</form>
So this way, everyone is getting redirected to their own personal page with their personal information that they can update.