How To Edit Customer Accounts to Add Birthdays

[TigsStore]

Hi, 

I am trying to find a way to edit the code of my website to allow customers to add their birthdays to their accounts.

I have two issues at this moment:

1. I have set up the birthday box for new accounts to enter their birthday but it does not save to their account information that they can see. I can see it on my admin side, but it does not permanently stay on their account that they can view.

2. I want my existing customers to be able to add their birthdays to their current accounts. I have edited the code to allow for the birthday box to appear. However, I am not sure how to save the information to their account or to my admin side of their account. 

I have viewed the other discussions pages and did not find a solution so I am hoping someone can help me. 

Thank you so much!!

Harris