There is no direct upgrade from JasperReports 7.9 to 9.0. There are two options:
1. Upgrade from 7.9 to 8.x to 9.0
2. Perform a fresh install of Jasper 9.0 and export/import the entire repository
What's the difference between an upgrade and a new installation of JasperReports Server?
SDM 17.x
JasperReports Server 7.9 to 9.0
The difference between an export/import and an upgrade is that the upgrade allows you to do an in-place upgrade for the existing schema. If you do a separate deployment, you'll have to have either a different DB node or a different schema name which may require additional actions from your side.