Editing php

Hello,

I've seen many instructions about doing this and that to the configuration of Atahualpa using the edit php function. I've been looking where I can use this edit function but can't find it anywhere in my wp-admin page, or in the appearance atahualpa themes secion of the admin page. Can you please tell me where to find this so I can do some stuff you've suggested to configure atahualpa. I'm using Atahualpa version 3.4.9.

Also, if I upgrade to the latest Atahualpa version 3.5.1, will I lose some of what i've already done to my site?

Many thanks for any help.

Easy Hiker