Acronis True Image is offered in 1, 3 and 5 seats packs.
To upgrade from any number of seats to 3 or 5 seats, use method 1, described below. To upgrade from 1 to 2 seats, and from any number of seats to 4 seats, use method 2.
Method 1 (recommended): Open "Account" tab in the product, sign into your Acronis account and click "Upgrade" / "Buy more". You will see the available upgrade options for your current license. The new multi-seat license will supersede (replace) your current license.
Method 2: Buy an additional license at Acronis website. Standard ("perpetual") license adds up more seats to your current license. Subscription license users: visit http://account.acronis.com after purchasing, click "Resolve conflict" and select the option with more seats, click "Resolve" button to confirm.