Liferay provides default upgrade scripts to make it easier to upgrade to new versions of Liferay.
Go to the ext/sql directory and run ant build-db. This wil automataically upgrade your database.
Note: If you have modified Liferay's default tables, you may need to customize the upgrade script according to your implementation. Upgrade scripts are located in the ext/sql/update-* folders.
If you auto-generated classes using service.xml, you will need to paste in the new version of the DTD and generate your classes again.