Hi,
if you are using Google's birthday calendar, then Google is loading them from your contacts on
https://contacts.google.com and you can edit them there or remove their birthday. Afterward you might have to re-download the birthday calendar using "Settings > Debug Tools > Calendars > Birthdays > Re-download" in CalenGoo.
If you are using CalenGoo's birthday calendar (i.e. if "Settings > Display and Use > General > Show birthdays of contacts" is turned on) then it is loading the birthdays from your phone's contacts. You can edit them in the "Contacts" app. Afterward you might have to turn "Settings > Display and Use > General > Show birthdays of contacts" off and on again to let CalenGoo reload your contacts.
Please don't hesitate to contact me again if you have further questions!
Kind regards
Dominique Gunia