Can an existing blank field be changed to that it defaults to Y, but can only be changed to N

I am a complete novice, I hope my terminology is correct. I have a database created long ago. It helps keep track of visitors. The visitor’s data is entered manually, but some fields are calculated and most cannot be empty. There is a field (“group2”) which is unused. I want to start using it. I want it to be pre-filled in by default with “Y”,but be able to change it to “N” (and no other characters and not empty). In my research, it looks like the ENUM function is usable to limit input; I wonder if I am on the right track, and if so, how do you set the pre-filled default to Y?