Maybe there is a program you could do this with, but I just changed my layout directly in the code.

You can go to the folder
/usr/share/X11/xkb/symbols
There you will find a ton of documents with short names. All those contain all the different keyboard layouts you can select when you open the "keyboard"(might be included in some other options application depending on your linux distribution) application.
Now you need to find the document where your layout is described in.
For English this would be either "gb" or "us".

You need to open the document as administrator!

In the files there are several regions encapsuled with {}. Those are the individual layouts you can select. You can go through the names(defined with something like name[Group1]="English (UK)";) until you find your layout.
There you can edit your layout.

The layouts are defined similar to this:
key <AE02> { [ 2, quotedbl, twosuperior, nabla ] };
Or more general:
key <some key code> { [ key, shift+key, alt gr+key, shift+alt gr+key ] };
If you don't know this key code for a key, you can look at the included layout to find it.

As you can see there is a special name for most of the characters(like 2 for 2, or nabla for ∇). If you don't know this name, you can search your character in the internet. There you might find a certain code associated with that character(like U03B1 for α) and enter that instead.

You might need to restart your computer after you did it, but I'm not sure.