You can upgrade from SQL Server 2012 to SQL 2019.

• Before upgrading SQL Server, enable Windows Authentication for SQL Server Agent and verify the default configuration: that the SQL Server Agent service account is a member of the SQL Server sysadmin group.
• To upgrade to SQL Server 2019 (15.x), you must be running a supported operating system. For more information, see Hardware and Software Requirements for Installing SQL Server.
• Upgrade will be not occurred if there is a pending reboot.
• Windows Installer service must be running to start installing.
• The SQL 2012 must be running SP4 and fully supported, If not install the SQL SP4 on SQL 2012 instance.
  
  

Preparation Task

 Use the Data Migration Assistant (DMA) to scan your databases to make sure they can be migrated to the 2019 server. It will highlight any potential blocking issues such as deprecated features that you may need to resolve before you can migrate. Make a list of deprecated features and discuss with the application owner if those features are still in use and then plan the upgrade as per your organization’s need.

UpgradeTask
 
The upgrade for databases is quite a simple process and you need to follow the instructions below.

• Perform a simple SQL Server Database Backup on the SQL 2012 Server instance using SSMS or with Transact-SQL command BACKUP
• Copy the database backup (*.BAK file) to the new SQL 2019 Server instance
• Restore the database using either SQL Server Management Studio (SSMS) or using the Transact-SQL command RESTORE DATABASE command.
  

During the restoration of the database on the new instance, it is automatically converted into the newer database version.

 If you restore a SQL Server 2012 or higher database to SQL Server 2019 (15.x), the database is automatically upgraded. Typically, the database becomes available immediately.