Tvheadend

If you want to purge it absolutely, completely, including-the-config-delete, then it's sudo apt-get purge tvheadend; this will remove the application and all associated configuration files. You can then install afresh and start over.

If you want to trash your config manually - or, indeed, back it up out of the way - then it's in /home/hts/.hts/tvheadend. If you stop tvheadend (sudo service tvheadend stop) and delete that directory then you're back at Day Zero. Restarting tvh will re-create a 3.4 config, or you can install 4.x and start with a clean config there instead. Note that deleting this directory will also delete the tvheadend superuser, so you may need to do a sudo dpkg-reconfigure tvheadend to recreate that.

However, you don't need to do this, and can install over the top, which means there's a fighting chance that much of your config (not all) will get migrated. All of this depends on whether you've done anything with the 3.4 installation yet, of course.

In essence, though, you simply need to follow the instructions here: https://tvheadend.org/projects/tvheadend/wiki/AptRepository and add the "Tvheadend - Stable branch - official releases" PPA.

Because the package name hasn't changed, your system will see the new PPA, see the new version, see that it's a higher number, and offer it as part of an apt-get update/apt-get upgrade, just like any other update. If you want to remove any doubt, though, look for any old PPA references in /etc/apt/sources.list, and comment them out (they'd have addresses something like http://apt.tvheadend.org/stable).