Looks like the PersonProperties class only provides readonly access as all the properties only show Get. MSDN PersonProperties
If you want to stay within SharePoint, it looks like you'll need to check out the UserProfile class. That page has a decent example of retrieving an account and then setting some properties.
If you don't need SharePoint specific properties and want an easy to use format you could retrieve the UserPrincipal. It will give you easy access to common user properties.
using (var context = new PrincipalContext(ContextType.Domain, "domainServer",
"DC=domain,DC=local", adminAccount, password))
{
var userPrincipal = UserPrincipal.FindByIdentity(context,
IdentityType.SamAccountName, "targetAccountName");
if (userPrincipal != null)
{
userPrincipal.GivenName = "NewFirstName";
// etc, etc.
}
}