You must have fully completed building the Television Tower to be able to begin upgrades, but you'll find that the first step is relatively simple: just add two additional staff members (read: ask your friends to help you) to your Television Tower and you'll upgrade not only the look of the building, but also the population cap. To be specific, the building will then support 500 citizens. Just remember that you have to ask neighbors that aren't already staff members in your Television Tower for this upgrade, as you can't hire the same person to work more than one job in the same building.
Once this first upgrade is done, the next upgrade isn't as smooth of sailing, requiring you to add 10 additional crew members, bringing you to the total of 22. You'll be able to earn staff members through individual friend requests, and once you have a friend working in the building, they won't show up again for requests. At least in this way, you'll know which friends to personally ask if you're having trouble finishing it off. Reach the Level 3 Television Tower, and you'll have an allowance of 900 citizens in your town, which isn't bad, if you have enough friends to help you reach that point. Good luck!
What do you think of the raised stats of the upgraded Television Tower? Should it support more than 900 citizens? Do you upgrade all of your city's other buildings? Let us know in the comments.