This depends on the details of what version you're looking to upgrade from and upgrade to.
One common approach is to download the new version, outfit it with your changes, skin, etc., migrate your data into the new environment, and roll over onto the new environment.
uPortal 2.1 comes with a database upgrade tool to upgrade from uPortal 2.0. See convert.xml which is an Ant build file that contains upgrade targets.
Unfortunately, there are not yet similar tools to automate upgrading an existing uPortal installation to uPortal 2.2. The project needs volunteers to write such tools and to share their upgrade experiences and insights.
Whatever upgrade you're trying to do, it's likely that someone has done something similar and would be willing to discuss.