You can't edit anything in the dayz_code.pbo. dayz_code is Client. If you want to modify anything from dayz_code you need to move it to your mission and edit the paths for the file. Or add an additional path to overwrite some things. Depends on if it is a function or not.
Let's say you want to have a custom compiles.sqf, so you can edit a lot of paths. It would be stupid to copy the whole file from your dayz_code.pbo to your mission since it would heavily increase the file size and just be useless since all paths are already done in the original file. Instead you make a custom one and only overwrite the paths you need. How do you do this? Go to your mission.pbo and create a dayz_code folder inside. Inside the dayz_code folder, create an init folder. In the init folder, create a compiles.sqf. Now you have a custom compiles.sqf where you can put your custom paths in. But for now, this file isn't called from anywhere, that's why you have to go to your init.sqf and add the file path for it below the original one so it loads the original one first and only overwrite those that are mentioned in your custom compiles.sqf.
A bit more tricky is editing a function because functions aren't split in variables. In one huge variable so you can't just overwrite things. Instead you move the original function (like fn_selfActions.sqf) from your dayz_code.pbo into your mission and replace the original path with the new one since the old one is no longer used.
All in all, never edit the dayz_code.pbo if you aren't an Epoch Dev.