I downloaded an add on scenery and it works fine. But i wanted to change and replace it's runway textures with another runway textures

 

What is the files responsible for runway textures in add on scenery texture file??

 

 

 

That will depend entirely on how the scenery file(s) is constructed.

If the runway texture is defined in the AFCAD using default textures, simply editing the airport layout file (AFCAD) will allow the texture to be changed, but be aware that airport layout details can be both "kept separate", as with a traditional AFCAD file, which can be edited with AFCAD2 or other similar utility, or incorporated in an overall scenery file. This latter option will need more sophisticated editing if other detail is not to be lost.

Some airfields are designed with "invisible" runways and presented using photographic representations … these photographic representations would need to be individually edited to provide the required appearance.