There is a not a simple step by step answer for you unfortunately as there is so much information we don't know about your hosting and Joomla environment. The approaches available will general fall into either the update-in-place or clone/copy which I think is the better way to go if it is available to you as be sure your updated site works without impact the current site.
You obviously have Akeeba Backup so taking a backup, or two, with that product is probably the first thing you can do and download at least one backup to somewhere other than your the default directory inside your Joomla installation. Getting familiar with the backup and restoring of Joomla instances using Akeeba is something you really should learn how to do and be comfortable with as you may need to recover your Joomla site if something goes wrong.
On your hosting environment manager(cPanel,Plesk) you will need to understand whether you can have multiple versions of PHP used at the same time, eg. PHP 7.2 and PHP 8.0. Regardless try restoring a backup into a different directory in your hosting account, essentially giving you a clone/copy of your Joomla instance, that you work with to update the extensions and even the core Joomla as you don't mention what version you are on. If the clone/copy of your site can use PHP 8 without impacting your current site then that is the better way to go.
If you aren't comfortable with the cloning/copy approach then you can follow the update-in-place method where you back up your site, update one or two of your extension to a later level that supports PHP 8, test that they work and update a couple of more extensions and test again, How often you might like to take a backup during this process is up to you as it will move your fallback/recovery position forward and you wont have to go back to the start if something goes wrong.
Once the extensions & Joomla versions are up to date then back up the site of course. From here you can use the function of your Hosting account manager to increase the PHP level by either going straight to PHP 8 or stepping up through 7.3 and 7.4, testing the site as you go. If problems arise you should be able to simply fallback to the previous PHP version until you can resolve the issue.
Which approach you take also depends on the usage of your website. For example a low usage website with little traffic and tolerable users you might be able to do the update-in-place and if there are odd messages or the site is down for a little while why you restore it nobody will complain. Conversely a high traffic site is probably not going tolerate messages and downtime so the clone/copy approach would be the better approach for you.
