Can't upgrade SDL Trados Studio 2019 Freelance Plus to 2021 Freelance Plus

Hi I've purchased SDL Trados Studio 2021 Freelance Plus. I've already upgraded 1 license, but there is still 1 SDL Trados Studio 2019 Freelance Plus, where I can't find any "upgrade" option. How can I upgrade it into 2021 version as well? thanks a lot.