You can use Travel Vouchers to unlock a City from inside your Passport. To do so, simply navigate to that City's Passport page, then select the green button that says "Use Travel Vouchers to unlock now!."
Just so you know, there are three different unlock options to choose from:
- 48 Hour Access, which costs 400 Travel Vouchers.
- 14 Day Access, which costs 2,000 Travel Vouchers.
- Permanent Access, which costs 4,000 Travel Vouchers.