I've annual subscription but want to include additional seats with monthly subscription how?

I’ve annual subscription but want to include additional design seats to my professional plan with monthly subscription how?

I’ve added 5 more seats but they are added as annual and the notification said it will be charged in the next billing. There is no page to downgrade the seats. The user experience in your billing services are absolutely super frustrating and time consuming.

Can someone please urgently help?

Thanks