The upgrade instructions are in the release notes. It looks long but is pretty straightforward if you haven't installed software from outside the debian ecosystem or created a frankenstein mix of packages from non-stable, non-backports distro versions.
I've installed vs code by downloading a .Deb, not an apt repo. Does that count as "software Frankenstein"? Follow up question, what's the worse that can happen? Vscode will break, or might the os get corrupted?
The Frankenstein stuff usually involves mixing package repos. Software distributed as a stand-alone .deb is generally pretty self-contained. I personally usually leave them alone. If you're following along with the upgrade instructions, it will have you remove "obsolete" packages. "Obsolete" packages are those which are not referenced from any currently configured package repo. So manually installed packages will show up there. You can decide whether to keep them or not when reviewing that list.
https://www.debian.org/releases/stable/i386/release-notes/ch...