Migrate azure sql database to Azure SQL Database Managed Instance
Since the Managed Instance was in preview, the first option we have is to export your Azure SQL Database as a bacpac then import that bacpac to your Managed Instance.

A better option we have now is using the Azure Data Migration service. It is not only faster than the method I described above, but it gives you the option to perform a data validation at end and it rebuilds all indexes on the database. Here you have a step-by-step tutorial.