Generally, technical tasks in the PB are frowned upon because they very rarely directly deliver business value to the customer. That's why User Stories are popular, because they force you to think about the business value of the story, and who it's being delivered to.
So, why are you upgrading the database? Can you identify business value in upgrading it, and why should the Product Owner agree to let you upgrade the database instead of building new features?
Is it because of a new feature that will make it possible or make it easier to do something in your application? In that case, that something should be the PB item, and the database upgrade should be a task within that story. If you already have stories on the PB that would benefit from the upgrade, then you should increase the estimates for one or more of those stories, and add the upgrade as a technical task to the story.
Is it because the vendor of the database is cutting off an old version from support? In that case you could have the upgrade as the story; something like, "As the department manager, I want to be sure that we have support for all of the software so that the continuity of the business isn't at risk if something goes wrong". Even that's pushing it, though. Generally, this kind of reason isn't really part of a project, unless the project has been going on so long the system software goes off support.
Is it for performance? Then the story should be about some aspect of the performance of the application that needs to be improved to deliver business value. Something like, "As a CSR I need to be able to retrieve customer information in a reasonable time so that customers on the phone are satisfied with our service". Then the upgrade becomes a task under that story.
Is it for some totally technical reason? If you can't identify how the upgrade is going to deliver business value, then why would you do it? Why would the Product Owner select it for a Sprint?