How to update legacy_root after running drush command

I've ran the command:
drush migrate-upgrade --legacy-db-key=mykey --legacy-root=/somedir --configure-only

I want to know how I can change the legacy root after I've already run the command, because re-running the command will create more config entities.

I don't need to update any of the migration config entities, just the one piece of configuration where the legacy-root is stored.

I can't seem to find where that is.

Thanks for your help!