This access to the 6 months or 12-month subscription depends entirely on the OS that you’re using. If you’re on Android, I’m sorry to say, but your only option is to run with the monthly subscription, however, if you’re on iOS, it changes slightly.
If you are indeed on iOS, you can change your subscription by first subscribing on the monthly plan, then going into the App Store → Account → Subscriptions → Infinite Flight, and choosing your subscription option from there.
Just a word of warning though, if you do opt to purchase the 6-month and 12-month subscription, you’ll only be able to see that reflect on your account after the end of your first month.