You would need to do a manual upgrade. It does need to be mentioned, we no longer support this, so if you are unsure, it may be best to get assistance. However if you want to take a look yourself, the process is
Backup fully
Upload a fresh set of files
Upgrade your server to PHP 8
Ensure things match the requirements checker - https://invisioncommunity.com/files/file/7046-invision-community-requirements-checker/
Run the upgrader from /admin/upgrade