Go to your Settings page and you will see the 1-click upgrade and downgrade option. To upgrade, you need to be out of the 7-day trial.
I don't see the upgrade option?
This is because you're still on the trial period. once your trial expires, the upgrade and downgrade option will be available. If you don't want to wait that long, contact support and they'll help hasten the process.
Why am I unable to downgrade my plan?
Your account needs to meet the required number of blogs in order for the downgrade option to appear.
If you're on Blogger Plan (15 blogs) and want to downgrade to our Mini Plan (10 blogs), make sure your account has 10 blogs or less - remove the extra blogs.