Birthdays in Contacts
On my iMac in Contacts Birthdays are grayed out and can't be edited
iMac 27″, macOS 10.15
Question marked as
you have not told us what steps you've tried, but:
when you click "edit" the birthday field is shown greyed out.
you then click the "+" sign and choose "birthday"
when you do that, the birthday entry will then be editable.
have fun.