There's two ways of doing this, and it all depends on which name is the most commonly credited name.
If he's more commonly known as Brandon Smith, then you need to find a birth year for either or preferably both Brandon Smiths. You enter it when you're in Edit Profile. Select the relevant Brandon Smith on the left-hand side of the list, then click "Edit Cast Member" below. This will give you a window where you can add the birth year. If you want to contribute this to the online database then I would recommend you list at least 3 different sources for the birth year.
If however he is more commonly credited as Brandon Mychal Smith then you do it a different way. In the profiles where he is credited as Brandon Smith, you link the name "Brandon Mychal Smith" to the role, then in the Edit Role window you will see an extra field called "Credited As", normally this field just duplicates the actor's name, but in this case you can edit it and remove the middle name.
Once saved you will see Brandon Smith in the profile's cast list, but that entry will now also link to all profiles also containing Brandon Mychal Smith.
Again, if you want to contribute online you will need to supply documentation that Brandon Mychal Smith is his most commonly credited name. The Credit Lookup tool can help here - you'll find a link to it via the Database page above.
Hope this helps, and I apologise for any inaccuracies - I'm at work and writing this from memory!