There’s no separate “upgrade”. Solo and practice are the same kind of account with a different number of people in it.
Solo to practice
Press Invite a colleague next to your tabs, or go to the Team tab, and add someone. Once you’ve sent an invite, the Team tab stays on your dashboard. When they join, your subscription goes to two seats.
Practice to solo
Remove everyone else from the Team tab. When you’re the only person left and there are no invites waiting, your dashboard goes back to the simple view and your bill to one seat.
Renaming your practice
Your practice name appears on invites. On the Team tab, press Rename next to the practice name. To change the name on your invoices too, update it under Manage billing on the Billing tab.