The baseband is updated when you update the firmware. Some firmware versions don't include baseband updates. The baseband version is only relevant for people with GSM (AT&T in the US) phones who rely an unlock so they can use an unofficial carrier like T-Mobile. You have a CDMA phone and you're with an official carrier so you don't need to worry about the baseband at all.
If your phone is Factory Unlocked, baseband version doesn't matter. Upgrading your baseband isn't going to give you any new features or anything like that, it's just a security upgrade to prevent users from software unlocking their phones. Sprint is a CDMA network so your baseband won't matter much, unless you're selling your phone, which it may be worth while to get factory unlocked by Sprint anyways.