If you have Paratext installed, and want to reset your registration code (or perhaps it has been reset if you have changed your organization), I find there is one annoying bug in Paratext that can result in it thinking the new code is invalid.
When you generate a new code, then start Paratext and go to Registration Information and paste the new code in, it seems that you must restart Paratext after entering the new code. Or else when you go to send/receive projects, it will tell you the new code is invalid.
So remember, if you paste in a new code, restart Paratext.
If you forget, and Paratext says your new code is invalid, you can fix it this way.
Close Paratext, then open Registry Editor. Look for Computer\HKEY_CURRENT_USER\SOFTWARE\Paratext\PreviousRegistrationCode
Delete the PreviousRegistrationCode value, then start Paratext and it should accept the new code.