Modifying bridges is a very complex process, one that cannot be done by using a simple app downloaded from the net in 5 minutes.
Once the bridges are in game, modifying them means doing modifications on the source material, which the authors don't release, and
then porting it all back in game.
It includes knowing how to use 3d apps (max), adobe photoshop for textures, and knowing how to port it in game, which includes knowledge of scripting for BC. This is a process only a handful of people were able to do so far, and even then rarely was one person able to do everything, as I understood it.
Even if you 'just' want to modify the textures, since they were lightmapped, you'd be in for a rough ride doing anything but minor adjustments. If you're interested in bridge modding, I suggest trying to learn to model in 3d, uvw map and texture...
as for the scripting part, you could either try to learn that yourself, or get help from someone, but first, you need to know how to actually make the bridge model and textures before you can do anything with them.