Hi,

1. I suggest doing that first on test.subdomain
2. Read: https://www.joomshaper.com/blog/how-to-update-your-site-to-joomla-4
3. All extensions (in particular plugins) and template must be updated. And if you are not sure, disable few.

Extra tips BEFORE!

• If you have already SPPB 3.8x there you can keep it. And ignore Joomla warnings.
• If you have PHP 7.4 - set PHP 8.0 , but not higher before J! upgrade.
• Set showing errors to Maximum in Global Configuration before Upgrade.

In general, Joomla upgrade is always risky. And request time, patience and well-rested mind (!)

I wish you luck :)