Upgrading to Liferay Portal CE 7.1 consists of two main steps: upgrading your installation and then upgrading the database. To upgrade to the latest release directly, you must be coming from Liferay Portal 6.1 or higher.
If you’re on Liferay Portal 6.0 or below, you should upgrade to Liferay Portal 6.2 before approaching an upgrade to the Liferay Portal CE 7.1 platform. Please see the Upgrading to Liferay Portal 6.2 article for information on upgrading to Liferay Portal 6.2 first.
Before you do anything, however, you should follow the pre-upgrade process of cleaning and normalizing your data. After that, you can prepare your system for the upgrade, and then finally run the upgrade process.
The tutorials that follow lead you through those steps.