I want to edit birthday congratulation email
I don't believe that the default phpBB3 installation sends out such and E-mail which would mean you have probably installed a modification to achieve this

So to end the E-mail you are going to find where the modification added the E-mail template, all the default ones are inside the /language/en/email/ folder and can be edited using a plain text editor so you may want to start by checking this location to see if you have a file relating to the modification.