The minimum .NET Framework version for NServiceBus.SqlServer version 6 is .NET 4.6.1.
All projects must be updated to .NET Framework 4.6.1 before upgrading to version 6.
It is recommended to update to .NET Framework 4.6.1 and perform a full migration to production before updating to version 6. This will help isolate any issues that may occur.
For solutions with many projects, the Target Framework Migrator Visual Studio extension can reduce the manual effort required in performing an upgrade.