RTF is a global modification that replaces all towns in the game.

A cunning one! :D Well, alright, everything's fair. 1) Extract the texture you need from the data. 2) In the properties, remove the "Read-only" restriction. 3) Open it with Paint.NET. 4) Edit and save it. 5) Then, you need to recreate all the folders. 6) Create a compressed ZIP folder. 7) Change the .zip extension to .pak. 8) Place it in the data folder.
Example:
Let's say I want to edit the hero icons. I go to:
1) data.pak - UI - H5A2 - ICONS - HEROES - HERO_NAME.dds
2) I extract the HERO_NAME.dds file. 3) I edit it.
Then, I create the folders UI, H5A2, ICONS, HEROES, and create the path as in point 1, except for data.pak. I create a compressed ZIP folder, put the UI folder inside, change the .zip to .pak, and give it a name, for example, MOD.pak.
I think I explained it clearly.