Since your question isn't related to the root, should be on the Lineage ThreadHi, I ask this:
on my LG V20 H910, I would like to switch from LineageOS 20 to LineageOS 21:
1. Can I upgrade or do I have to do a new installation?
2. If I can upgrade, what is the procedure?
3. If I can't update and have to do a new installation, do I have to delete everything? In this case, how do I delete to install LineageOS 21?
1) Yes you can upgrade, but if you consider that even many manufacturers made official upgrades and some users or scenarios don't go as smooth. You should consider whenever possible do a clean/fresh upgrade. (I know it's a pain, troublesome etc, but in the possibility if you have any trouble, you don't know unless you start fresh).
2) In the official documentation has this section Upgrade to a higher version of LineageOS. Isn't that complicated, just read with time and can work, but again, read the 1st answer.
3) My personal recommendation: Use Neo Backup and make the backups for the apps (usually some apps that require some accounts may not work, you should login again, but at least the apk it's correctly installed). Transfer everything to your computer > Follow the instructions to a clean install (specially the erasing/formatting everything > Install Neo Backup again and transfer the folder back to your device > Restore every app > Be happy