The direct upgrade to v. 1.28 is only available for v. 1.27 instances running Ubuntu 20. Customers running earlier versions of SwaggerHub On-Premise must first upgrade to v. 1.27 (including migration to Ubuntu 20) and then upgrade to v. 1.28.
The links below provide detailed upgrade instructions for various base versions of SwaggerHub On-Premise.
From 1.18 or earlier
Upgrading from v. 1.26 and earlier requires activating your license again as part of Ubuntu 20 migration. You should have the license file in a prior email from SmartBear or saved locally. If you do not have a local copy of the license file, contact SmartBear Support.