In your buffer, M-x yas-visit-snippet-file, then type the name of the snippet you want to edit, or choose from a prompted list. Then it will open a buffer for you to edit. From the buffer you can M-: buffer-file-name to see the path of the file containing the snippet.
You can delete chars from the buffer during expansion, but modifying the buffer during expansion is dicouraged.
Deleting chars is pretty easy, because yasnippet allows to eval elisp code during expansion. You have to put this code into back-quotes Read more about it here.
Your snippet would then look like this example (you need to refine this crude example,...