itunes won't launch
went to back up my iphone -- getting a new one and itunes would not launch.
Don't know what's wrong.
tried to download and it says I have latest version.
Your post isn't clear. Messages telling you that you need to install the latest version appear in iTunes, so if you have seen that message, then presumably, iTunes did launch.
If iTunes will not see your phone, and you've got as far as iTunes telling you that you have the latest version, that suggests that you have also seen a message telling you that you need the latest version of iTunes to back up the phone.
However, this usually indicates that you have a Windows 7 or Windows 8 machine, running iTunes 12.10.11.2, but the latest version of iTunes (on the day of writing this) is 12.12.7.1. Apple's message about this is misleading, as you have perhaps found out.
If you require any more information, please confirm which version of iTunes you're running and which version of Windows. In addition, please post the precise wording of the message you see.