You select how long you want the contract to run for.
It's anything from 3 months to 36 months.
Are you viewing these via the app?
If so they don't tend to show all the info and offers available (as I found out last night when checking my upgrade options)
I find checking in the shop via a PC gives you far more info.
There again I rarely use my phone to check for upgrades.
*The Game Is On*