If you're not jailbroken - wipe your phone and supervise it with Apple Configurator 2 (on macOS, free), or with iMazing (on Windows, might be paid).
Already have data already? No problem. Back up your phone, wipe and prepare, update, then restore backup.
DON'T BACKUP IF YOU'RE JAILBROKEN
Restoring a backup will unsupervise your phone (unless you made the backup while supervised), so make sure you update before you restore your backup.
This is personally what I used, as I had a new in box iPhone SE with 13.4 installed.
more on how to do this later