Hi Cezar,
According to your tag, you are using Office 365 pre-upgrade. If it is right, this article you provided is for Office 365 after upgrade. It’s not suit for you, we have a specific support channel for such issue. I suggest you call support for billing and subscription issues to get more help about your situation.
If you are using Office 365 after upgrade, since you are using Exchange Online Plan 1, you can refer to the steps for Office 365 Small Business to use the Switch plans wizard.
Thanks,
August