To migrate data from your existing NoSQL database, you need to download an external tool called Raven.Migrator.
The application is available at ravendb.net/downloads, in the TOOLS package (see image below).
The Raven.Migrator tool can be found in the downloaded Tools folder.
Raven.Migrator Tool Download
Import from MongoDB
In the RavenDB Studio client on the left side, select Tasks -> Import Data.