Well yes – it can actually be done. You would need to create your alternate template and then in the appropriate place switch to your template if the topic belonged to a specific forum. Actually very easy to achieve once the template is created.

Is there any instructions how to do this anywhere?

No! This is the first time we have been asked for anything like this. We could talk you through it though. But I think the first thing you would need to do would be to design and decide what your alternative template would look like compared to the normal one you are using.