If you made your original purchase from prison-architect.com you need to look here.
If you bought a tier from steam, but wish to upgrade to a higher tier you can do so.
1. Check that we haven't sold out by visiting prison-architect.com
2. Make a paypal donation of the price difference between your current tier and the one to which you wish to upgrade.
eg. If you have bought the Base Pack for $30 and wish to get your name in the game ($50) you need to make a donation of $50 - $30 = $20.
3. Find the location of your steam profile page:
a. Login to Steam. Select Account (top right) -> View Profile -> Edit My profile (left)
b. Add a custom url (it'll look like http://steamcommunity.com/id/kingdarwinian/)
c. Check the settings and make it public (so we can check you bought a tier)
4. Mail your paypal receipt for the donation together with your steam profile address to firstname.lastname@example.org.
That's it! We'll upgrade you and you'll receive an email when it's been done.