even if I updated Enfold to v7, I do now have 3 Themes Installed: 1. Enfold Child. 2. Enfold v7, Enfold v4.9
How can I get rid of the old theme in the Manager? Can I just delete v4.9 or could this be risky?
You need to override the old version with 7.0 — deleting the old version and renaming the latest one should work. To be safe, please make sure to create a site backup or restore point before proceeding.