[e12e]

> Now, how do I upgrade?

Read the documentation, follow it.

https://www.debian.org/releases/bookworm/amd64/release-notes...

> Just apt upgrade?

Mostly (please do RTFM):

  # update to latest point release of current system:
  sudo apt update
  sudo apt upgrade
  sudo apt full-upgrade
  sudo apt --purge autoremove

  # Update sources list
  sudo sed -i 's/bullseye/bookworm/g' /etc/apt/sources.list

  # upgrade to bookworm:
  sudo apt update
  sudo apt upgrade --without-new-pkgs
  sudo apt full-upgrade
  sudo apt --purge autoremove

> Does it matter that I have i3wm instead of whatever the default is?

Generally no, unless you've specified/use third party (or to a lesser extent "back port") software sources.