Is the version of IPB you’re running now compatible with PHP8? If not, you’ll need to do a manual upgrade.
Disable all 3rd party applications & plugins.
Switch to a default non-customized IPS theme.
Switch your server to PHP 8.
Use the compatibility checker script to confirm you have all of the required PHP modules installed in your PHP8.
Manually upload the files of the new version from your client area to your server.
Visit yourdomain.com/admin/upgrade and follow the prompts to do the upgrade.
Check if any 3rd party modifications require an update (there's a button to manually check in the applications page). After you apply the updates you can safely re-enable those modifications.
Go to your Support Page in ACP and fix any remaining issues listed on the page.
If you still have disabled applications and/or plugins after the above steps, re-enable them 1 by 1 to make sure they are compatible with PHP 8.