Backup your data first.
If anything goes wrong you can restore it right away and try again.
Select the People tab.
Select Quick Update.
On this next screen you may remove anyone from the list that you do not want to change their envelope number.
On this final screen you may select how you want to assign the numbers. Do NOT clear all Envelope Numbers for all persons first if you have deleted anyone from the list because you want them to keep their old number.