IMPORTANT: If you're upgrading from Calendar 1.x on ExpressionEngine 2.x, it's recommended that you upload the new Calendar files to your new ExpressionEngine 3.x site directory just before running the EE2 -> EE3 upgrade. Please see Calendar 1.x to 2.x Upgrade Guide before considering upgrading.
Calendar has its own Update Service built in, which means that every time there's an update available, an Updates Available nav item will show up in the Calendar add-on area of the EE control panel. You can then review the changelog there, or view it here.
To download the update, simply click the Get Latest Version button, and you'll be taken to your account area on the Solspace site where you can log in and download the updated package for Calendar. Then follow the steps below: