is it possible to edit an entry from the front-end when you're not logged in? I'm working on a checkout process for a takeaway restaurant where anonymous visitors can place orders using guest entries.

In this stage the order is saved with a status of "unpaid". Once a visitor saves the order he's taken to a payment provider service where he pays for his order, once the order is payed the visitor is redirected to a page where the order status would be set to "paid" in a hidden form.

In order to accomplish this the aforementioned saved guest entry would have to be edited from the front-end. Is it possible to anonymously edit an entry?
You should be able to edit that entry using craft()-&gt;entries-&gt;saveEntry($entry) with $entry-&gt;authorId set to a registered user with the necessary permissions. To get the ID of the user that created the entry, access the Guest Entries plugin settings in your plugin:

$settings = craft()-&gt;plugins-&gt;getPlugin('guestentries')-&gt;getSettings();
$authorId = $settings-&gt;defaultAuthors[$entry-&gt;section-&gt;handle];