If you mean updating Julia’s minor version, say, jill install 1.7.0-rc1, then you probably would want to cp -r ~/.julia/environments/v1.6 ~/.julia/environments/v1.7 (if you’re using macOS or Linux) so that the package information is still kept in the new version’s root environment @v1.7. Actually, jill install 1.7.0-rc1 --update also does this but I find manually copying the folder via cp is more convenient so I choose to not advertise this feature in the README.
Deleting Julia via jill is a missing feature: currently, you’ll have to manually remove the Julia installation folders and also the symlinks, see also the discussions in https://github.com/johnnychen94/jill.py/issues/96.