Please refer to our plugin installation documentation located here: https://ilcdocs.atlassian.net/wiki/spaces/ILCFP/overview
You can export the custom field data .CSV before making any updates with the Mass Apply Plugin if you’d like to revert your changes by re-importing them manually.
Currently there is no way to mass apply line item level Custom Fields, but it is in the development plan.
Currently there is no way to mass delete Custom Fields, but it is in the development plan.